JP2014170715A - Cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To further restrict energy efficiency decrease in an aqueous cell containing sulfur.SOLUTION: A cell according to the invention comprises: a positive electrode; a negative electrode; and an aqueous electrolyte in which an alkali metal polysulfide is dissolved, as an active material, which is y≥3 in a composition formula MS(M represents alkali metal) reduced most by charging or discharging. A cell according to the invention may be redox flow cell 10, which uses electrolyte as a positive electrode and/or negative electrode. The redox flow cell 10 includes: a case 12; a separator 18 separating the inside of the case 12 into a positive electrode chamber 14 and a negative electrode chamber 16; a positive electrode collector plate 20 arranged in the positive electrode chamber 14; and a negative electrode collector plate 22 arranged in the negative electrode chamber 16. The aqueous electrolyte may be distributed by being accommodated in the positive electrode chamber 14 or negative electrode chamber 16.

Description

  The present invention relates to a battery, and more particularly to a battery that charges and discharges by circulating an aqueous electrolyte.

  A redox flow battery, which is a type of secondary battery, is a fluid battery that performs charging and discharging by causing an oxidation-reduction reaction of an active material dissolved in an electrolytic solution to proceed by circulating the solution. Vanadium-based, ZnBr, ZnCr batteries and the like have been put into practical use. Among these, vanadium-based batteries have characteristics such as high cycle characteristics, but the raw materials are scarce and it is desired to use raw materials that are more abundant. Although ZnBr batteries and ZnCr batteries have relatively abundant raw material resources in nature, there is a problem of dendrite generation at the Zn electrode, and in order to prevent this, an operation of regularly performing full discharge is necessary. A battery using sulfide ions such as sodium sulfide as an active material has abundant raw material resources in nature, and is promising in terms of ease of manufacture and cost reduction. As a system using sulfide ions as a negative electrode active material, a sodium sulfide-bromine battery has been studied as a demonstration plant. There are also reports of flow batteries combined with sodium ferrocyanide cathodes. On the other hand, batteries using sulfide ions as the positive electrode active material, for example, zinc as the negative electrode, are also promising battery systems such as abundant raw material resources and cost reduction.

  For example, an aqueous electrolyte solution in which a water-soluble sulfide such as sodium sulfide is dissolved as a positive electrode active material will be described as an example. When charging / discharging, a two-stage discharge characteristic is exhibited depending on the oxidation state of sulfur. Among them, there is a problem that very large polarization occurs in charge and discharge in a low oxidation state, and energy efficiency of charge and discharge is lowered. For example, on the other hand, what reduces overvoltage by carrying | supporting cobalt on carbon as a catalyst is proposed (for example, refer nonpatent literature 1).

Electrochemica Acta, 51, 6304 (2006)

  However, in the battery of Non-Patent Document 1 described above, cobalt is supported on carbon as a catalyst to reduce overvoltage. However, cobalt is not abundant in resources, is costly, and has catalyst degradation. Needed a solution. That is, it has been desired to further suppress a decrease in energy efficiency of charge / discharge. In addition, although the battery using a water-soluble sulfide as a positive electrode active material was taken as an example, even in a battery using a water-soluble sulfide as a negative electrode active material, the phenomenon at the time of discharging only occurs at the time of charging, which is an essential difference. There is no.

  This invention is made | formed in view of such a subject, and it aims at providing the battery which can suppress the fall of energy efficiency more.

  As a result of diligent research to achieve the above-described object, the inventors of the present invention have found that in a battery using an aqueous electrolyte solution in which a water-soluble sulfide is dissolved, the most reduced state is obtained by increasing the sulfur concentration in the electrolyte solution. For example, it has been found that a high oxidation state can be maintained even during charging when used as a positive electrode and during charging when used as a negative electrode, polarization can be greatly reduced, and a decrease in energy efficiency can be further suppressed. It came to complete.

That is, the battery of the present invention activates a positive electrode, a negative electrode, and an alkali metal polysulfide in which y ≧ 3 in the composition formula M ₂ S _y (where M is an alkali metal) in the most reduced state by charging or discharging. And an aqueous electrolyte solution dissolved as a substance.

The battery of the present invention can further suppress a decrease in energy efficiency. The reason why such an effect is obtained is presumed as follows. For example, in a sulfide battery, the polarization changes greatly depending on the oxidation state of sulfur. For example, in a battery in which sodium sulfide is dissolved in a water-based electrolyte as a positive electrode active material, a high voltage is applied in a high oxidation state (low reduction state) of Na ₂ S ₄ or more, and a low oxidation state of Na ₂ S ₃ -Na ₂ S ( In the high reduction state), it exhibits a two-stage discharge characteristic depending on the oxidation state of sulfur, which is a low voltage. Here, if it can be used only in a highly oxidized state, it leads to a reduction in polarization. For example, as the reduction proceeds, a counterion of a sulfide anion generated by the reduction is required. Therefore, the oxidation state of sulfur can be controlled by limiting the amount of alkali metal ions that are counterions. Alternatively, the amount of active material at the counter electrode can be limited, and control can be performed so that the sulfur oxidation state of the polysulfide is in a highly oxidized state even in the most reduced state. Here, by limiting the Y ≧ 3 in the most reduced composition formula M ₂ S _y states, polarization greatly reduced, thereby improving the energy efficiency.

FIG. 3 is an explanatory diagram showing an outline of the configuration of the redox flow battery 10. The charge-discharge measurement result of the comparative example 1. The measurement result in each electric current which charged / discharged the comparative example 1 in the high voltage range. The measurement result in each electric current which charged / discharged the comparative example 1 in the low voltage range. The relationship figure of the charging / discharging voltage difference with respect to electric current value. The measurement result in each electric current which charged / discharged the comparative example 2 in the low voltage range. The charging / discharging measurement result in each electric current of Example 1. FIG. FIG. 6 is a relationship diagram of a discharge voltage with respect to a normalized capacity in which the discharge capacity is normalized to 1. The charge / discharge measurement result of Example 3.

The battery of the present invention includes a positive electrode, a negative electrode, and an aqueous electrolyte. This aqueous electrolytic solution dissolves, as an active material, an alkali metal polysulfide in which y ≧ 3 in the composition formula M ₂ S _y (where M is an alkali metal) in the most reduced state by charging or discharging. The battery of the present invention may be a redox flow battery using an electrolyte for the positive electrode and / or the negative electrode. The battery includes, for example, a separator that separates the case into a positive electrode and a negative electrode, an electrode chamber that houses the positive electrode or the negative electrode, and a liquid feeding unit connected to the electrode chamber. It is good also as what is accommodated and distribute | circulated by the liquid feeding part. At this time, the aqueous electrolyte may be accommodated in the positive electrode chamber in contact with the positive electrode and distributed through the liquid feeding unit. Alternatively, the aqueous electrolyte may be accommodated in the negative electrode chamber in contact with the negative electrode and distributed through the liquid feeding unit. The positive electrode and the negative electrode here are determined by the potential difference between the two types of electrodes. If the electrode using the aqueous electrolyte of the present invention has a noble potential with respect to the counter electrode, the positive electrode will be a negative electrode. . For example, the former example is a zinc-sodium sulfide battery, and the latter example is a sodium sulfide-bromine battery. Hereinafter, the redox flow battery will be specifically described.

In the battery of the present invention, the aqueous electrolyte is dissolved by using, as an active material, an alkali metal polysulfide in which y ≧ 3 in the composition formula M ₂ S _y (where M is an alkali metal) in the most reduced state by charging or discharging. doing. Discharge here is a case where polysulfide is used as a positive electrode active material, and charging is a case where it is used as a negative electrode. In addition, since the distinction between the positive electrode and the negative electrode is determined by the potential difference from the counter electrode, there is no essential difference between using as the positive electrode or the negative electrode. This aqueous electrolyte solution is more preferably dissolved in an alkali metal polysulfide having y ≧ 3.3 in the above composition formula, and more preferably y ≧ 3.5. In the range of y ≧ 3.3, energy efficiency can be further increased. From the viewpoint of energy density, y ≦ 5 is preferable. Examples of the alkali metal contained in the alkali metal polysulfide include sodium, lithium, potassium, and the like. Among these, sodium rich in resources is more preferable.

In the battery of the present invention, the aqueous electrolyte solution may be prepared so that alkali metals and sulfur as active materials are included in the range of x> 4 in the composition formula M ₂ S _x . This way, easily and y ≧ 3 in the most reduced composition formula M ₂ S _y states, it is possible to further suppress a decrease in energy efficiency. In this composition formula M ₂ S _x , x ≧ 5 is preferable, and x ≧ 6 is more preferable. “The aqueous electrolyte is prepared in the range of x> 4 in the composition formula M ₂ S _x ” means a so-called charge amount of alkali metal and sulfur in the aqueous electrolyte of the present invention. At this time, from the viewpoint of not using unnecessary sulfur, it is preferable that x ≦ 10 in this composition formula. In addition, since the alkali metal may be contained in the aqueous electrolyte solution of the counter electrode, the relationship of x ≧ y is established between the charged composition M ₂ S _x and the composition formula M ₂ S _y at the time of the most reduction.

The alkali metal polysulfide has a composition formula M ₂ S _x (M is an alkali metal), and includes sodium sulfide, lithium polysulfide, and potassium potassium sulfide in which sulfide ions are in the form of polysulfide ions. In the alkali metal polysulfide, the value of x can be easily adjusted by adding sulfur to an alkali metal sulfide such as sodium sulfide, lithium sulfide, or potassium sulfide. The amount of alkali metal polysulfide dissolved in the aqueous electrolyte is not particularly limited, but is preferably 5% by mass or more, more preferably 10% by mass or more, and more preferably 15% by mass or more with respect to the aqueous electrolyte. More preferably. This aqueous electrolytic solution dissolves alkali metal polysulfides, but does not need to dissolve all of the sulfide active material, and may be in a partially precipitated state in a saturated state.

  In the battery of the present invention, the aqueous electrolyte is preferably neutral and alkaline, and may be an alkaline solution in which sodium hydroxide, potassium hydroxide, etc. are dissolved, or the pH is adjusted from neutral to alkaline using a buffer. It is good also as the buffer solution controlled to the area. It is not preferable that the aqueous electrolyte is acidic because generation of hydrogen sulfide is a concern. The aqueous electrolyte solution preferably has a pH of 9 or more. The aqueous electrolyte is preferably an alkali hydroxide solution of 0.1 N or more and 4 N or less, for example.

  In the battery of the present invention, the electrode includes an electrode current collector that contacts the aqueous electrolyte. The electrode current collector is not particularly limited as long as it can exchange electrons with an active material (alkali metal polysulfide). For example, carbon blacks such as ketjen black and acetylene black, and scaly graphite It may include graphite such as natural graphite, artificial graphite, and expanded graphite, and conductive fibers such as carbon fiber and metal fiber. The electrode current collector may be a metal such as platinum, copper, silver, nickel, or aluminum, or may be an organic conductive material such as a polyphenylene derivative. Alternatively, a hole through which the aqueous electrolyte solution can permeate may be provided in the electrode current collector, and electrons may be exchanged between the electrode current collector and the aqueous electrolyte solution through this hole.

  In the battery of the present invention, the negative electrode in the case where an aqueous electrolyte solution in which an alkali metal polysulfide is dissolved is used for the positive electrode, a material that is electrochemically stable in the potential range when the negative electrode active material to be used is charged and discharged is used. Are preferable, for example, a metal plate of zinc. At this time, the aqueous electrolyte solution of the negative electrode can be an aqueous solution according to the material of the negative electrode. For example, the aqueous electrolyte solution may contain zinc ions such as zincate ions, or may be suspended and saturated with zinc oxide. . The negative electrode and the aqueous electrolyte solution for the negative electrode may be appropriately selected in consideration of battery performance.

  In the battery of the present invention, the positive electrode in the case where an aqueous electrolytic solution in which alkali metal polysulfide is dissolved is used as the negative electrode is, for example, an aqueous electrolytic solution in which alkali metal bromide is dissolved or an aqueous electrolytic solution in which ferrocyanide alkali metal is dissolved. Etc. can be used for the positive electrode electrolyte. Examples of the alkali metal bromide include lithium bromide, sodium bromide, and potassium bromide. Examples of the alkali metal ferrocyanide include lithium ferrocyanide, sodium ferrocyanide, and potassium ferrocyanide. What is necessary is just to select these alkalis suitably according to a negative electrode electrolyte solution.

  In the battery of the present invention, the separator is not particularly limited as long as it has a composition that can withstand the use conditions of the battery. For example, an ion conductive polymer film that can conduct ions or an ion conductive solid electrolyte film is used. Can do. Examples of the ion conductive polymer membrane include a perfluorocarbon material (tetrafluoroethylene-perfluorovinyl copolymer) composed of a hydrophobic tetrafluoroethylene skeleton composed of carbon-fluorine and a perfluoro side chain having a sulfonic acid group. ) And the like. Examples of the ion conductive solid electrolyte membrane include cation conductive glass (oxide glass).

  In the battery of the present invention, the liquid feeding unit may be a liquid feeding pump, for example. The aqueous electrolyte solution for the positive electrode and / or the negative electrode may be circulated in a predetermined amount using a liquid feed pump, and the predetermined flow rate can be appropriately set according to the battery scale. In addition, the liquid feeding unit may include a circulation path connected to the positive electrode chamber, the liquid feeding pump, and the electrolytic solution tank, and circulate the positive aqueous electrolyte solution. In addition, the liquid feeding unit may include a circulation path connected to the negative electrode chamber, the liquid feeding pump, and the electrolyte tank, and circulate the aqueous electrolyte solution of the negative electrode.

FIG. 1 is an explanatory diagram showing an outline of the configuration of the redox flow battery 10. The redox flow battery 10 includes a case 12, a separator 18 that separates the inside of the case 12 into a positive electrode chamber 14 and a negative electrode chamber 16, a positive current collector 20 disposed in the positive electrode chamber 14, and a negative electrode chamber 16. The negative electrode current collector plate 22 was provided. In the redox flow battery 10, a positive electrode side circulation path 26 is provided between the positive electrode chamber 14 and a positive electrode aqueous electrolyte tank 24 that stores the positive electrode aqueous electrolyte, and a positive electrode side circulation pump is provided in the middle of the positive electrode side circulation path 26. 28 is attached. On the other hand, a negative electrode side circulation path 32 is provided between the negative electrode chamber 16 and the negative electrode aqueous electrolyte solution tank 30 for storing the negative electrode aqueous electrolyte solution, and a negative electrode side circulation pump 34 is attached in the middle of the negative electrode side circulation path 32. Yes. In the redox flow battery 10, a case where an aqueous electrolyte solution in which an alkali metal polysulfide is dissolved is used as a positive electrode will be described as an example. The positive aqueous electrolyte solution contains a composition formula M ₂ S _{y in the} most reduced state. (Wherein M is an alkali metal), an alkali metal polysulfide satisfying y ≧ 3 is contained as an active material. The positive electrode-side circulation pump 28 circulates the positive electrode aqueous electrolyte and the negative electrode-side circulation pump 34 circulates the negative electrode aqueous electrolyte so that charging and discharging are performed. In addition, when combining with a more noble electrode such as using a bromine-based electrolytic solution, a redox flow battery having the same configuration in which the aqueous electrolytic solution of the present invention is used as a negative-electrode electrolytic solution can be obtained.

The battery of the present invention described in detail above includes an aqueous electrolyte solution in which an alkali metal polysulfide having y ≧ 3 in the composition formula M ₂ S _y in the most reduced state (where M is an alkali metal) is dissolved. Therefore, energy efficiency can be further increased. The reason why such an effect is obtained is that, for example, when the reduction of the alkali metal polysulfide proceeds, a counterion of a sulfide anion generated by the reduction is required, but the amount of the alkali metal ion that is a counterion is limited. ing. For this reason, the oxidation state of sulfur can be controlled. For example, since it can be used only in the high oxidation state with little polarization among the two-stage discharge characteristics, the reduction in energy efficiency can be further suppressed.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  For example, in the above-described embodiment, the redox flow battery has been described. However, the present invention is not particularly limited thereto, and the electrolytic solution may not be circulated. Even in this case, since the aqueous electrolyte contains a sulfur compound that is an active material and a water-soluble compound, energy efficiency can be further improved.

  In the above-described embodiment, the other aqueous electrolyte solution is made to circulate also for the counter electrode of the electrode having the aqueous electrolyte solution of the present invention. However, the present invention is not particularly limited thereto, and the counter electrode is configured not to circulate the aqueous electrolyte solution. Also good.

  Hereinafter, an example in which the battery of the present invention was specifically manufactured will be described as an example.

[Evaluation cell]
A redox flow battery shown in FIG. 1 was produced. The configuration of the cell will be described below. A zinc metal plate was used as the negative electrode, and a 0.5N sodium hydroxide aqueous solution in which zinc oxide was suspended and saturated was used as the aqueous electrolyte solution for the negative electrode. A carbon-impregnated carbon felt was used as the positive electrode current collector, and a 0.5 N sodium hydroxide aqueous solution containing a predetermined concentration of sodium sulfide was used as the aqueous electrolyte solution of the positive electrode. The electrode was 2.5 cm square. A cation exchange membrane Nafion N324 (registered trademark) was used as a diaphragm separating the aqueous electrolyte solution of the positive electrode and the aqueous electrolyte solution of the negative electrode. Using the redox flow battery of FIG. 1, the positive and negative electrode electrolytes are each fed at a rate of 3.3 ml / min using a roller pump, the current is 200 mA, charging for a predetermined time, resting for 30 seconds, charging for a predetermined time discharge. The discharge operation was performed, and then the same charge / discharge operation was performed at a current of 50 mA, in the order of 200 mA, 50 mA, 20 mA, 10 mA, 5 mA, 2 mA, 1 mA, and the charge and discharge waveforms were measured. In order to examine the effect of the value of y when using M ₂ S _y (where M is an alkali metal), the negative electrode was evaluated using a zinc metal plate so that the potential was constant, but other negative electrodes were used. be able to. Further, as the counter electrode, an alkaline metal bromide aqueous solution, a sodium ferrocyanide aqueous solution, or the like can be used as the positive electrode electrolyte, and an aqueous electrolyte containing an alkali metal polysulfide can be used as the negative electrode electrolyte. Hereinafter, details will be described in comparative examples and examples.

[Comparative Example 1]
A battery using a 0.5N sodium hydroxide aqueous solution containing ₂ % by mass of sodium sulfide (Na ₂ S) was defined as Comparative Example 1. FIG. 2 shows the charge / discharge measurement results when charging / discharging was performed with the upper limit voltage during charging being 1.2V and the lower limit voltage during discharging being 0.1V. As shown in FIG. 2, a two-stage waveform was shown during discharge. FIG. 3 shows the result of repeated charging and discharging for 10 minutes while changing the current in a high oxidation state (for example, a high voltage region of 0.8 V or higher in FIG. 2) after fully charged. FIG. 4 shows the result of repeated charge and discharge in a low voltage range of less than 0.8 V at 2). In the case of high current, the voltage rises or falls within 10 minutes. Therefore, in charge / discharge in the high voltage range, charge / discharge is performed with the upper limit voltage during charging being 1.3 V and the lower limit voltage during discharging being 0.6 V. It was. Moreover, in charging / discharging in a low voltage region, charging / discharging was performed by setting the upper limit voltage during charging to 1.1V and the lower limit voltage during discharging to 0.3V. As shown in FIG. 3, it was found that the charge and discharge in the high voltage region has a small polarization except when the charge and discharge is performed at a high current of 20 mA. Further, as shown in FIG. 4, it was found that in any charge / discharge in the low voltage region, the polarization is large at any current. FIG. 5 shows the current dependency of the charge / discharge voltage difference obtained from FIGS. As shown in FIG. 5, it was found that the polarization is small in the high voltage range, but the polarization increases in the low voltage range.

[Comparative Example 2]
A battery using a 0.5N sodium hydroxide aqueous solution containing 10% by mass of sodium sulfide was defined as Comparative Example 2. FIG. 6 shows the result of charging / discharging in the low voltage region while changing the current. Even when the concentration of sodium sulfide was high, the polarization was large. FIG. 5 shows the current dependency of the charge / discharge voltage difference. As shown in FIG. 5, it was found that polarization was large at any current during charge / discharge in the low voltage range.

[Example 1]
Sulfur was mixed with sodium sulfide so as to have a charged composition of Na ₂ S ₆ (Na ₂ S _x x = 6), and this was dissolved in a 0.5N aqueous sodium hydroxide solution, and 10% by mass of Na ₂ S ₆ was added. A battery was produced in the same manner as in Comparative Example 1 except that the 0.5N aqueous sodium hydroxide solution was used. Using the battery of Example 1, constant current charging / discharging was performed at 5 mA for 10 minutes from the initial state. Next, the battery was charged to 1.2 V at 50 mA and then discharged to 50 V at 0.1 V. Further, after discharging to 0.1 V, the current was changed to respective currents of 50 mA, 20 mA, 10 mA, 5 mA, 2 mA and 1 mA, and the work of charging and discharging was repeated for 10 minutes. FIG. 7 shows the results of charge / discharge measurement at each current in Example 1. FIG. 5 shows the current dependency of the charge / discharge voltage difference of Example 1. As shown in FIGS. 7 and 5, Example 1 showed the same small polarization as in the high voltage range of the battery containing Na ₂ S of Comparative Example 1 even after discharge. That is, it has been found that the reduction in energy efficiency of charge / discharge can be further suppressed.

[Example 2]
A battery produced in the same manner as in Example 1 except that an aqueous solution having a charged composition of Na ₂ S ₅ (Na ₂ S _x x = 5) was used was designated as Example 2.

[Example 3]
A battery produced in the same manner as in Example 1 except that an aqueous solution having a preparation concentration of 20% by mass was used was designated as Example 3. In this aqueous electrolyte solution, the whole amount was not dissolved at a high concentration of 20% by mass, and a part of solid was precipitated.

[Comparative Examples 3 to 6]
Batteries prepared in the same manner as in Example 1 except that the charged composition was Na ₂ S ₄ , Na ₂ S _3.5 , Na ₂ S ₃ and Na ₂ S ₂ , respectively, and an aqueous solution having a concentration of 10% by mass was used. It was set as Comparative Examples 3-6. Using these batteries, measurement of charging and discharging at a constant current of 5 mA for 10 minutes in the initial state, measurement of charging at 50 mA to 1.2 V and discharging to 0.1 V at 50 mA, and 10 minutes after discharging at 5 mA Measurements were performed with constant current charging and discharging.

(Results and discussion)
The composition ratios of Examples 1 and 2 and Comparative Examples 3 to 6 with the most advanced sulfur reduction were determined from the polysulfide concentration of the positive electrode and the sodium ion concentration in the negative electrode electrolyte, and are shown in Table 1. . Table 1 also shows the initial value of the charge / discharge voltage difference measured at a current of 5 mA in Examples 1 and 2 and Comparative Examples 3 to 6 and a current of 5 mA after discharging at 50 mA to 0.1 V (after discharge). The values were also shown. As shown in Table 1, in Comparative Examples 3 to 6, the charge / discharge voltage difference after discharging to 0.1 V at 50 mA showed a large value. On the other hand, in Examples 1 and 2, even after discharging at 50 mA to the initial state and 0.1 V, it was found that the small charge / discharge voltage difference was maintained, and the polarization was further reduced. As in Example 1, Na ₂ S ₆ having a charged composition of Na ₂ S _x (x> 4) has a high voltage in a high oxidation state (low reduction state) of Na ₂ S ₄ or higher, and Na ₂ S ₃ − In the low oxidation state (high reduction state) of Na ₂ S, it is considered that a two-stage charge / discharge characteristic with a low voltage is exhibited (see Comparative Example 1). In Examples 1 and 2, since it was the composition Na ₂ S _y in the most reduced state that did not shift to the low oxidation state, it became clear that polarization that could occur in the low oxidation state did not occur. In Comparative Example 3, the voltage difference in the initial state was as small as about half compared to the other comparative examples. From this, it was inferred that there was a suitable composition boundary between Comparative Example 3 and Example 2 which were relatively good. From the above, Comparative Example 3 in which the feed composition is Na ₂ S ₄ is the boundary, and in the most reduced state, Na ₂ S _y has y of 2.8 or more, and further y is 3 or more. It was found to be effective.

FIG. 8 is a relationship diagram of the discharge voltage with respect to the normalized capacity obtained by normalizing the discharge capacity of Example 1 and Comparative Examples 2 and 5 to 1. When the discharge waveforms shown in FIG. 8 are compared, Example 1 shows a one-stage waveform in which the plateau (voltage flat portion) is maintained at a high voltage around 0.7 V, and then the voltage drops to the lower limit voltage. Comparative Example 5 showed a two-stage discharge behavior similar to that of FIG. 2, which was high at about 0.7 V in the middle and then decreased to about 0.5 V. In Comparative Example 2, the discharge state continued to a lower voltage. As described above, in Example 1, a high voltage, that is, a state in which the polarization is small is shown in almost all discharge regions, whereas in the comparative example in which _y of Na ₂ Sy is small, the discharge becomes low as the discharge proceeds. The discharge in the voltage range, that is, a large polarization state was shown. Therefore, the comparative example leads to a decrease in energy efficiency. As shown in this example, if _y of M ₂ S _y (M is an alkali metal) is 3 or more with the most advanced sulfur reduction, a high voltage range (high oxidation) can be obtained without external control. It was found that the state can be maintained.

  The results of charging and discharging at 50 mA using the battery of Example 3 are shown in FIG. As shown in FIG. 9, it was found that the two-stage waveform was not formed during discharge, and it was found that the same result as in Example 1 was obtained even in a state where the solution was not a uniform solution and excessive sulfur was added. .

In this example, the state in which the reduction is most advanced is determined by the amount of sodium on the negative electrode side, the sulfur concentration of the polysulfide on the positive electrode side, and the oxidation state. Therefore, as a result of the shortage of sodium in the counter ion, the reduction of sulfur does not proceed any more, and even in the state where the reduction of sulfur is the most advanced, it remains in a highly oxidized state of M ₂ S ₃ or more, and as a result, even if the discharge proceeds It is thought to be smaller. In this example, since a large excess of zinc was used as the negative electrode, the oxidation state of the polysulfide during the most reduction was determined by the amount of sodium in the negative electrode side electrolyte, the amount of sulfur on the positive electrode side, and its redox state. However, if the amount of zinc is limited, the oxidation state of the polysulfide at the time of reduction is determined by the amount of zinc, not sodium. Further, for example, when Na ₂ S-bromine polysulfide electrolyte is used for the positive electrode and polysulfide is used for the negative electrode, the amount of bromine in the positive electrode electrolyte, the amount of sulfur in the negative electrode electrolyte, and the oxidation-reduction thereof Depending on the state, the oxidation state of the polysulfide at the time of reduction is determined. In this way, the battery system is assembled so that the _y of the polysulfide composition M ₂ S _{y in} the most reduced state remains 3 or more when using an aqueous electrolyte containing an alkali metal polysulfide as either the positive electrode or the negative electrode. For example, the high voltage region (high oxidation state) can be maintained without external control such as drive voltage limitation. It is clear that the method can be realized by defining the amount of active material of the counter electrode or the amount of alkali metal ions in the counter electrode electrolyte, together with the concentration, amount and oxidation state of the alkali metal polysulfide. .

  The present invention is applicable to the battery industry.

10 redox flow battery, 12 case, 14 positive electrode chamber, 16 negative electrode chamber, 18 separator, 20 positive electrode current collector plate, 22 negative electrode current collector plate, 24 positive electrode aqueous electrolyte tank, 26 positive electrode side circulation path, 28 positive electrode side circulation pump, 30 negative electrode aqueous electrolyte tank, 32 negative electrode side circulation path, 34 negative electrode side circulation pump.

Claims (4)

A positive electrode;
A negative electrode,
An aqueous electrolyte solution in which an alkali metal polysulfide having y ≧ 3 in the composition formula M ₂ S _y (where M is an alkali metal) in the most reduced state by charging or discharging is dissolved as an active material;
With battery.   The battery according to claim 1, wherein the aqueous electrolyte solution dissolves the alkali metal polysulfide satisfying y ≧ 3.3 in the composition formula.   The battery according to claim 1 or 2, wherein the aqueous electrolyte contains sodium as the alkali metal. The battery according to any one of claims 1 to 3,
A separator for separating the positive electrode and the negative electrode;
An electrode chamber containing the positive electrode or the negative electrode;
A liquid feeding part connected to the electrode chamber,
The battery in which the aqueous electrolyte is accommodated in the electrode chamber and is distributed by the liquid feeding unit.