Method for prolonging cycle life of ferrous sulfide redox flow battery

Abstract

The invention belongs to the technical field of flow batteries, and provides a method for prolonging the cycle life of a ferrous sulfide redox flow battery, and the method specifically comprises the following steps: (1) drawing an absorbance curve of a standard ferrous sulfide redox flow battery as a standard curve; (2) taking a to-be-detected ferrous sulfide redox flow battery, and drawing an absorbance curve of which the SOC is lower than that of the standard ferrous sulfide redox flow battery; and (3) when the peak value of the absorbance curve of the ferrous sulfide redox flow battery to be detected at the characteristic absorption wavelength of the polysulfide ions is lower than that of the standard curve, replacing the electrolyte on the sulfur side of the battery with a fresh electrolyte. According to the ultraviolet determination method for the concentration of the sulfur-iron battery electrolyte, special instruments are not needed, the operation process is simple and rapid, assistance of other chemical reagents is not needed, the analysis result is accurate and reliable, and the effect of prolonging the service life of the battery is achieved by supplementing active ions in the sulfur-side electrolyte.

Description

Technical Field

[0001] The present invention belongs to the technical field of flow batteries, and particularly relates to a method for extending the cycle life of a polysulfide-iron redox flow battery. Background Art

[0002] With the rapid development of renewable clean energies such as wind energy and solar energy, large-scale energy storage systems have attracted increasing attention in the market. Among them, flow batteries, as a new type of electrochemical energy storage technology, have the characteristics of high safety, long service life, low system cost, decoupling of energy and power, etc., and are therefore recognized as the most promising energy storage technology candidates. However, the currently most commercially promising all-vanadium flow battery has a high installation cost due to the price limitations of the electrolyte and separator materials. At the same time, its low energy density also limits the battery capacity.

[0003] Polysulfides are widely used in various flow battery systems due to their high solubility and low cost. At the same time, ferricyanides have also been widely studied due to their high chemical activity, good reversibility, and low chemical cost. In recent years, polysulfide-iron flow batteries constructed using polysulfides and ferricyanides have gradually come into the public eye and shown excellent commercial prospects. In a flow battery, the electrolyte serves as the energy storage unit, and the content of active ions in the electrolyte affects the utilization rate of the electrolyte, thereby affecting the capacity and utilization rate of the battery system, and further affecting the life of the battery system. However, due to the shuttle effect of polysulfide ions and the inevitable airtightness problems of the device system, the active ions in the sulfur-side electrolyte are easily oxidized into other non-redox-active ions during long-term cycling, resulting in the inactivation of the electrolyte and affecting the battery cycle life. Therefore, how to solve the problem of shortened battery life caused by electrolyte inactivation is crucial. Summary of the Invention

[0004] The technical problem to be solved by the present invention is that the shuttle effect of polysulfide ions easily leads to electrolyte inactivation and shortened battery life. To overcome the above-mentioned deficiencies and defects in the background art, a method for extending the cycle life of a polysulfide-iron redox flow battery is provided.

[0005] To solve the above technical problem, the technical solution proposed by the present invention is as follows: A method for extending the cycle life of a polysulfide-iron redox flow battery, the polysulfide-iron redox flow battery being a polysulfide-iron redox flow battery composed of a polysulfide solution and a ferricyanide solution, the method comprising the following steps: (1) Perform a charge and discharge program test on a standard sulfur-iron redox flow battery, control its discharge state to a SOC of 10-50%, take the sulfur-side electrolyte for concentration detection by spectrophotometer method, and determine its absorbance curve as the standard curve; record the SOC value of this standard sulfur-iron redox flow battery as X; (2) Take the sulfur-iron redox flow battery to be tested, control its discharge state until the SOC is lower than the SOC of the standard sulfur-iron redox flow battery in step (1), take the sulfur-side electrolyte for concentration detection by spectrophotometer method, draw its absorbance curve, and record the SOC value of the sulfur-iron redox flow battery to be tested as Y, which should meet 1% <X-Y<20%; (3) When the peak value of the absorbance curve of the sulfur-iron redox flow battery to be tested at the characteristic absorption wavelength of polysulfide ions is lower than that of the standard curve, the electrolyte on the sulfur side of the battery is replaced with fresh electrolyte. Sulfur iron redox flow battery 2- / S x 2- and Fe(CN)6 2+ / Fe(CN)6 3+ It is a redox pair. When charging, an oxidation reaction occurs at the positive electrode, and the valence state of the active substance increases; a reduction reaction occurs at the negative electrode, and the valence state of the active substance decreases; the opposite is true during discharge. During each charge and discharge process, the sulfur element undergoes a redox reaction back and forth between -2 valence (sulfide) and -1 valence or 0.5 valence (polysulfide). Polysulfide ions easily pass through the diaphragm and are oxidized into sulfur powder on the iron side; and are easily oxidized by oxygen that penetrates into the electrolyte tank, causing an imbalance in the charge state of the electrolyte on both sides of the battery, resulting in inconsistent amounts of active ion substances on both sides, which ultimately leads to battery performance degradation and affects the battery cycle life.

[0006] In the sulfur-side electrolyte solution, since it contains a variety of polysulfide ions with the same maximum absorption wavelength, each polysulfide ion does not have a single characteristic peak, so it is impossible to find the specific relationship between absorbance and the concentration of a single ion. Therefore, it is necessary to establish a standard curve graph corresponding to the characteristic wavelength of polysulfide ions and absorbance, and compare the absorbance curve of the test solution with the standard curve graph to determine the concentration range of the polysulfide ions.

[0007] When the battery to be tested is at a slightly lower SOC state, the absorbance curve of polysulfide ions of a normal battery will correspond to or be slightly higher than the standard curve. When the measured curve is lower than the standard curve, it means that there are too few polysulfide ions in the electrolyte and the electrolyte needs to be replaced to repair the battery.

[0008] X and Y can take any value within the value range, but should satisfy Y < X. Therefore, X - Y needs to be greater than 1%, and the difference in values should not be too large. Thus, X - Y is restricted to be less than 20%. By restricting the discharge capacity, an accurate SOC value can be obtained, and the resulting curve has better measurement effects.

[0009] Preferably, the amount of active ionic substances in the sulfur-side electrolyte of the sulfur-iron redox flow battery to be measured should be greater than or equal to the amount of active ionic substances in the iron-side electrolyte, to ensure that the number of transferable electrons in the sulfur-side electrolyte is greater than or equal to the number of transferable electrons in the iron-side electrolyte.

[0010] Preferably, the concentration of active ionic substances in the sulfur-side electrolyte of the standard sulfur-iron redox flow battery is the same as that in the electrolyte of the sulfur-iron flow battery to be measured, and the amount of active ionic substances in its iron-side electrolyte is more than twice that in the sulfur-side electrolyte and above, to ensure that the number of transferable electrons in the iron-side electrolyte is greater than or equal to the number of transferable electrons in the sulfur-side electrolyte.

[0011] In the same battery, the amount of active ions in the negative electrode electrolyte needs to be greater than that in the positive electrode electrolyte, to ensure an excess of active ions on the sulfur side, so that the reaction stays at the conversion between disulfide ion S2 2- and tetrasulfide ion S4 2- .

[0012] Preferably, the polysulfide solution of the sulfur-iron redox flow battery contains one or more of sulfide ions, disulfide ions or tetrasulfide ions, the ferricyanide solution contains one or more of ferricyanide ions or ferrocyanide ions, and the electrolyte of the sulfur-iron redox flow battery includes one or more of NaCl, KCl, LiCl, NaBr, KBr, LiBr, NaOH, KOH or LiOH.

[0013] Preferably, in step (1), the concentration detection by spectrophotometry to determine its absorbance curve as the standard curve specifically includes: (1) Using spectrophotometry to determine the characteristic absorption wavelengths of polysulfide ions and supporting electrolytes in ultraviolet absorption; (2) Taking the sulfur-side electrolyte of the standard sulfur-iron redox flow battery in a certain state when the SOC is 10 - 50% and preparing it into a standard solution; using spectrophotometry for full-spectrum scanning, measuring the characteristic wavelength and absorbance of the solution, and drawing the ultraviolet absorption curve of the polysulfide ion wavelength and absorbance in this state as the standard curve; Preferably, in step (2), taking the sulfur-side electrolyte for concentration detection by spectrophotometry and drawing its absorbance curve specifically includes: Take the sulfur-side electrolyte of the sulfur-iron redox flow battery to be tested and prepare it into a standard solution; use the spectrophotometer method for full-spectrum scanning, measure the characteristic wavelength and absorbance of the solution, and draw the ultraviolet absorption curve of the polysulfide ion wavelength and absorbance in this state.

[0014] Preferably, the controlling of its discharge state in steps (1) and (2) specifically includes: performing constant-current discharge on the sulfur-iron redox flow battery, using the discharge capacity as the cut-off condition, and stopping the discharge when the calculated discharge capacity reaches a% of the theoretical total capacity. At this time, the SOC of the battery is 1 - a%.

[0015] Preferably, the determination of the characteristic absorption wavelengths of the polysulfide ions and the supporting electrolyte in ultraviolet absorption specifically includes: Prepare a standard sample solution of polysulfide ions with a concentration of <10 mmol / L from the sulfur-side electrolyte, perform full-spectrum scanning in the wavelength range of 200 nm to 900 nm, determine the characteristic absorption wavelength and solution absorbance of the polysulfide ions, and draw the ultraviolet absorption standard curve template with the wavelength as the abscissa and the absorbance as the ordinate.

[0016] Preferably, the characteristic absorption wavelength of the polysulfide ions is 300 nm.

[0017] Preferably, the supporting electrolyte of the sulfur-iron redox flow battery has no characteristic absorption wavelength at 200 - 900 nm.

[0018] More preferably, when preparing the standard sample solution, the purity of the standard sample used is above analytical purity.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The ultraviolet determination method for the concentration of the sulfur-iron battery electrolyte provided by the present invention does not require special instruments, and the operation process is simple and rapid. It does not need to rely on other chemical reagents for assistance, and the analysis results are accurate and reliable. By supplementing the active ions in the sulfur-side electrolyte, the effect of extending the battery life can be achieved; (2) The present invention judges whether the battery electrolyte is inactivated by detecting the concentration of active ions in the battery electrolyte by spectrophotometry. Specifically, by detecting the absorbance curve of the active ions in the test solution and the absorbance curve of the active ions in the standard battery, the effective concentration of the active ions in the electrolyte can be accurately judged. Therefore, this application can accurately judge the liquid change timing; (3) The method for extending the cycle service life of the sulfur-iron battery provided by the present invention solves the problem of battery performance attenuation caused by the inactivation of active ions in the electrolyte of the sulfur-iron battery. It can perform non-destructive repair treatment on the battery without disassembling the battery and damaging the main structure of the battery, extend the service life of the battery, and has extremely high industrial value. Description of the Drawings

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0021] Figure 1 For the full spectrum diagram of the 0.5 mM polysulfide ion (S 2- / S2 2- / S4 2- ) solution in the wavelength range of 200 - 900 nm in Example 1.

[0022] Figure 2 For the full ultraviolet absorption spectrum diagram of the sulfur - iron standard battery in the 25% SOC state in Example 1.

[0023] Figure 3 For the full ultraviolet absorption spectrum diagram of the sulfur - side electrolyte of the battery after 100 long cycles in Example 1.

[0024] Figure 4 For the full ultraviolet absorption spectrum diagram of the sulfur - side electrolyte of the battery after 252 long cycles in Example 1.

[0025] Figure 5 、 6 For the capacity and efficiency diagram of the battery after replacing the sulfur - side electrolyte after 252 cycles of the battery in Example 1.

[0026] Figure 7 For the full ultraviolet absorption spectrum diagram of the sulfur - side electrolyte of the battery after 100 long cycles in Example 2.

[0027] Figure 8 For the full ultraviolet absorption spectrum diagram of the sulfur - side electrolyte of the battery after 273 long cycles in Example 2.

[0028] Figure 9 、 10 For the capacity and efficiency diagram of the battery after replacing the sulfur - side electrolyte after 273 cycles of the battery in Example 2.

[0029] Figure 11 、 12 For the capacity and energy efficiency diagram of the battery during long - term cycling before and after liquid replacement in Comparative Example 1.

[0030] Figure 13 、 14 For the capacity and energy efficiency diagram of the battery after long - term cycling before and after liquid replacement in Comparative Example 2. Detailed implementation manners

[0031] For the convenience of understanding the present invention, the present invention will be described more comprehensively and in detail below in conjunction with the accompanying drawings of the specification and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0032] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0033] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present invention can be obtained through market purchases or can be prepared by existing methods.

[0034] Example 1 A method for extending the cycle life of a sulfur-iron redox flow battery. The charge and discharge mode of the sulfur-iron redox standard battery is constant current charge and discharge, and its current density is 80 mA·cm -2 , and specifically includes the following steps: S1-1 Use spectrophotometry to determine the characteristic absorption wavelength of polysulfide ions in the sulfur-iron redox flow battery: The supporting electrolyte of the sulfur-iron redox flow battery in this example is sodium hydroxide, and the concentration of sodium hydroxide is 1 mol / L. A scanning spectrum of the sodium hydroxide solution is made in the wavelength range of 200-900 nm to determine that the sodium hydroxide solution has no characteristic absorption peak in this wavelength range and will not interfere with the absorbance measurement of polysulfide ions.

[0035] For a 0.5 mM polysulfide ion (S 2- / S2 2- / S4 2- ) solution, a full-spectrum scan is performed in the wavelength range of 200~900 nm respectively to determine its peak position in the wavelength range of 200~900 nm. As Figure 1 shown, it is the full-wave spectrum diagram of a 0.5 mM polysulfide ion (S 2- / S2 2- / S4 2- ) solution in the wavelength range of 200~900 nm.

[0036] S1-2 Use spectrophotometry to determine the absorbance standard curve of the sulfur-side electrolyte of the sulfur-iron redox standard battery: Accurately prepare positive electrode iron-side electrolyte and negative electrode sulfur-side electrolyte with certain concentrations. The positive electrode electrolyte is a 0.5 M potassium ferricyanide + 1 M sodium hydroxide solution, and the negative electrode electrolyte is a 0.5 M Na2S2 + 1 M sodium hydroxide solution. Add them to the electrolyte tank according to the volume ratio of the positive and negative electrode electrolytes of 2:1.

[0037] The battery is discharged at a constant current, with the discharge capacity as the cut-off condition. When the discharge capacity reaches 75% of the theoretical capacity, the discharge stops. At this time, the SOC of the battery corresponds to 25%. Take 1 ml of the sulfur-side electrolyte, dilute it 1000 times with 1 M NaOH so that the final concentration is in the range of 0.1 - 1 mmol / L, and perform a full-spectrum scan on this solution to determine its corresponding absorbance.

[0038] Plot the absorbance of the obtained sulfur-side electrolyte, and obtain the absorbance-wavelength curve of the sulfur-side electrolyte of the battery in the state of SOC being 25%. The obtained curve graph is used as the standard absorbance curve of the sulfur-side electrolyte in the state of 25% SOC, as Figure 2 shown.

[0039] S1-3 Use spectrophotometry to determine the absorbance curve of the sulfur-side electrolyte of the sulfur-iron battery to be measured: For the sulfur-iron flow battery to be measured, its initial formulation is that the positive electrolyte is 0.5 M potassium ferricyanide + 1 M sodium hydroxide solution, and the negative electrolyte is 0.5 M Na2S2 + 1 M sodium hydroxide solution. Add them to the electrolyte tank according to the volume ratio of the positive and negative electrolytes of 1:1.2. After long-cycle testing, take the sulfur-side electrolyte of the sulfur-iron redox flow battery to be measured and prepare it into a sample solution to be measured. Perform a full-spectrum scan using spectrophotometry, measure its wavelength and absorbance, and plot the curve of the wavelength and absorbance of polysulfide ions.

[0040] For the sulfur-iron battery to be measured, after long cycling to 100 cycles, with the discharge capacity as the cut-off condition, when the discharge capacity reaches 80% of the theoretical capacity, that is, when the corresponding SOC is 20%, take 1 ml of the sulfur-side electrolyte, dilute the test solution 1000 times with 1 M NaOH, perform a full-spectrum scan using spectrophotometry, measure its wavelength and absorbance, and plot the curve of the wavelength and absorbance of polysulfide ions. Compare the wavelength and absorbance curve of the sample to be measured with the standard curve in the state of 25% SOC, as Figure 3 shown. The obtained absorbance curve is higher than the standard curve, so there is no need to replace the electrolyte of the battery.

[0041] For the sulfur-iron battery to be measured, after long cycling to 252 cycles, with the discharge capacity as the cut-off condition, when the discharge capacity reaches 80% of the theoretical capacity, that is, when the corresponding SOC is 20%, take 1 ml of the sulfur-side electrolyte, dilute the test solution 1000 times with 1 M NaOH, perform a full-spectrum scan using spectrophotometry, measure its wavelength and absorbance, and plot the curve of the wavelength and absorbance of polysulfide ions. Compare the wavelength and absorbance curve of the sample to be measured with the standard curve in the state of 25% SOC, as Figure 4 shown. The obtained absorbance curve is lower than the standard curve, so replace the sulfur-side electrolyte with a fresh initial-state electrolyte.

[0042] As Figure 5 , 6 shown, after the liquid replacement in the 252nd cycle, its capacity and energy efficiency can still be maintained at about 99% of the initial level in the subsequent charge-discharge cycles, without showing obvious performance degradation. Example 2 A method for extending the cycle life of a thionyl chloride redox flow battery. The charge-discharge mode of the thionyl chloride redox standard battery is constant current charge-discharge, and its current density is 80 mA·cm -2 , and specifically includes the following steps: S2-1 Use spectrophotometry to determine the absorbance standard curve of the sulfur-side electrolyte of the thionyl chloride redox standard battery: Accurately prepare the positive electrode iron-side electrolyte and negative electrode sulfur-side electrolyte with certain concentrations. The positive electrode electrolyte is a solution of 0.6 M potassium ferricyanide + 1 M sodium hydroxide, and the negative electrode electrolyte is a solution of 0.5 M Na2S4 + 1 M sodium hydroxide. Add them to the electrolyte tank according to the volume ratio of the positive and negative electrode electrolytes of 2:1.

[0043] Perform constant current discharge on the battery. Using the discharge capacity as the cut-off condition, stop discharging when the discharge capacity reaches 75% of the theoretical capacity. At this time, the SOC of the battery corresponds to 25%. Take 1 ml of the sulfur-side electrolyte and dilute it 1000 times with 1 M NaOH to make the final concentration within the range of 0.1 - 1 mmol / L. Perform a full-spectrum scan on this solution to determine its corresponding absorbance.

[0044] Plot the absorbance of the obtained sulfur-side electrolyte to obtain the absorbance-wavelength curve of the sulfur-side electrolyte of the battery at the SOC of 25%. The obtained curve graph is used as the standard absorbance curve of the sulfur-side electrolyte at the 25% SOC state, as Figure 7 shown.

[0045] S2-2 Use spectrophotometry to determine the absorbance curve of the sulfur-side electrolyte of the thionyl chloride battery to be measured: For the thionyl chloride redox flow battery to be measured, its initial formulation is that the positive electrode electrolyte is a solution of 0.8 M potassium ferricyanide + 1 M sodium hydroxide, and the negative electrode electrolyte is a solution of 0.5 M Na2S4 + 1 M sodium hydroxide. Add them to the electrolyte tank according to the volume ratio of the positive and negative electrode electrolytes of 1:1. After long-cycle testing, take the sulfur-side electrolyte of the thionyl chloride redox flow battery to be measured and prepare it into a sample solution to be measured. Use spectrophotometry for full-spectrum scanning, measure its wavelength and absorbance, and plot the curve of the wavelength and absorbance of polysulfide ions.

[0046] For the lithium-sulfur battery to be tested, after long cycling to 100 cycles, with the discharge capacity as the cut-off condition, when the discharge capacity reaches 80% of the theoretical capacity, that is, when the corresponding SOC is 20%, 1 ml of the sulfur-side electrolyte is taken, diluted 1000 times with 1 M NaOH, and full-spectrum scanning is performed by spectrophotometry to measure its wavelength and absorbance, and a curve of the wavelength and absorbance of polysulfide ions is plotted. The wavelength and absorbance curve of the test sample is compared with the standard curve at 25% SOC. As Figure 7 shown, the obtained absorbance curve is higher than the standard curve. Therefore, there is no need to replace the electrolyte of the battery.

[0047] For the lithium-sulfur battery to be tested, after long cycling to 273 cycles, with the discharge capacity as the cut-off condition, Figure 9 、 10 Figure 9 shows the capacity and efficiency of the battery after replacing the sulfur-side electrolyte after 273 cycles of the battery in Example 2; when the discharge capacity reaches 80% of the theoretical capacity, that is, when the corresponding SOC is 20%, 1 ml of the sulfur-side electrolyte is taken, diluted 1000 times with 1 M NaOH, and full-spectrum scanning is performed by spectrophotometry to measure its wavelength and absorbance, and a curve of the wavelength and absorbance of polysulfide ions is plotted. Figure 8 Figure 11 shows the ultraviolet absorption full-wave spectrum of the sulfur-side electrolyte of the battery after 273 cycles of long cycling in Example 2. The wavelength and absorbance curve of the test sample is compared with the standard curve at 25% SOC. As Figure 4 shown, the obtained absorbance curve is lower than the standard curve. Therefore, the sulfur-side electrolyte is replaced with fresh initial-state electrolyte.

[0048] As Figure 7 shown, after replacing the electrolyte, the battery capacity can still be maintained at the same level as the original in subsequent charge-discharge cycles, and the energy efficiency can also be maintained at about 90% of the initial level.

[0049] Comparative Example 1 This comparative example in this paper relates to a method for extending the cycle life of a lithium-sulfur redox flow battery, including the following steps: Imitating the lithium-sulfur redox flow battery under the same operating conditions in Example 1, its formulation is that the positive electrolyte is 1 M potassium ferricyanide + 1 M sodium hydroxide solution, and the negative electrolyte is 0.5 M Na2S2 + 1 M sodium hydroxide solution. Currently, ultraviolet spectrophotometric test analysis is not performed on the sulfur-side electrolyte of the battery, and the battery is only replaced with electrolyte at the 500th cycle when the battery performance decays severely. The results are as Figure 11 、 12 shown. After replacing the electrolyte, neither its capacity nor its energy efficiency can be restored to the initial level, and the subsequent decay is faster.

[0050] Comparative Example 2 This comparative example of the present invention relates to a method for extending the cycle life of a thionyl chloride redox flow battery, comprising the following steps: Imitating the thionyl chloride redox flow battery under the same operating conditions as in Example 2, the formulation is that the positive electrolyte is 1 M potassium ferrocyanide + 1 M sodium hydroxide solution, and the negative electrolyte is 0.5 M Na2S4 + 1 M sodium hydroxide solution. Currently, no ultraviolet spectrophotometric test analysis is performed on the sulfur-side electrolyte of the battery. Only at the 436th cycle when the battery performance decays severely, the battery is refilled with electrolyte. The results are as Figure 13 , 14 shown. After refilling, both its capacity and energy efficiency fluctuate greatly and fail to recover to the initial level, and the subsequent decay is faster.

[0051] In Examples 1 and Comparative Example 1, and Examples 2 and Comparative Example 2, since the electrolyte of the battery is detected before obvious decay of the battery performance in the examples, and the electrolyte is refilled before the active ions in the electrolyte begin to decrease but the performance has not changed significantly, the effective concentration of the active ions in the battery electrolyte is ensured, thereby extending the cycle life of the battery; while in the comparative examples, obvious decay has occurred in the battery, and then the electrolyte is refilled, which has damaged the root of the battery and cannot achieve the purpose of extending the battery life by refilling. Therefore, regularly checking the concentration of active ions in the electrolyte of the long-cycle battery to determine whether fresh electrolyte needs to be replaced is an effective means to extend the battery life and ensure the long-term operation of the battery.

Claims

1. A method for extending the cycle life of a sulfur-iron redox flow battery, characterized in that, The sulfur-iron redox flow battery is a sulfur-iron redox flow battery composed of a polysulfide solution and a ferricyanide solution. The method includes the following steps: (1) Perform charge-discharge program tests on a standard sulfur-iron redox flow battery, control its discharge state to an SOC of 10 - 50%, take the sulfur-side electrolyte for concentration detection by spectrophotometry, and determine its absorbance curve as the standard curve; record the SOC value of this standard sulfur-iron redox flow battery as X; (2) Take the sulfur-iron redox flow battery to be tested, control its discharge state to an SOC lower than that of the standard sulfur-iron redox flow battery in step (1), take the sulfur-side electrolyte for concentration detection by spectrophotometry, plot its absorbance curve, and record the SOC value of this sulfur-iron redox flow battery to be tested as Y, which should satisfy 1% < X - Y < 20%; When the peak value of the absorbance curve of the sulfur-iron redox flow battery to be tested at the characteristic absorption wavelength of polysulfide ions is lower than the standard curve, replace the sulfur-side electrolyte of the battery with fresh electrolyte.

2. The method according to claim 1, characterized in that, The amount of active ionic substances in the sulfur-side electrolyte of the sulfur-iron redox flow battery to be tested should be greater than or equal to the amount of active ionic substances in the iron-side electrolyte.

3. The method according to claim 1, wherein The concentration of active ionic substances in the sulfur-side electrolyte of the standard sulfur-iron redox flow battery is the same as that in the electrolyte of the sulfur-iron redox flow battery to be tested, and the amount of active ionic substances in its iron-side electrolyte is more than twice the amount of active ionic substances in the sulfur-side electrolyte.

4. The method according to claim 1, wherein The polysulfide solution of the sulfur-iron redox flow battery contains one or more of sulfide ions, disulfide ions or tetrasulfide ions, the ferricyanide solution contains one or more of ferricyanide ions or ferrocyanide ions, and the electrolyte of the sulfur-iron redox flow battery includes one or more of NaCl, KCl, LiCl, NaBr, KBr, LiBr, NaOH, KOH or LiOH.

5. The method according to claim 1, characterized in that, The specific steps of determining the absorbance curve as the standard curve by performing concentration detection by spectrophotometry in step (1) include: (a) Use spectrophotometry to determine the characteristic absorption wavelengths of polysulfide ions and supporting electrolytes in ultraviolet absorption; Take the sulfur-side electrolyte of the standard sulfur-iron redox flow battery in a certain state with an SOC of 10 - 50% and prepare it into a standard solution; perform a full-spectrum scan by spectrophotometry, measure the characteristic wavelength and absorbance of the solution, and plot the ultraviolet absorption curve of the polysulfide ion wavelength and absorbance in this state as the standard curve.

6. The method according to claim 1, wherein The specific steps of taking the sulfur-side electrolyte for concentration detection by spectrophotometry and plotting its absorbance curve in step (2) include: Take the sulfur-side electrolyte of the sulfur-iron redox flow battery to be tested and prepare it into a standard solution; perform a full-spectrum scan by spectrophotometry, measure the characteristic wavelength and absorbance of the solution, and plot the ultraviolet absorption curve of the polysulfide ion wavelength and absorbance in this state.

7. The method according to claim 1, wherein The specific control of its discharge state in steps (1) and (2) includes: performing constant current discharge on the sulfur-iron redox flow battery, using the discharge capacity as the cut-off condition, and stopping the discharge when the calculated discharge capacity reaches a% of the theoretical total capacity. At this time, the SOC of the battery is 1 - a%.

8. The method according to claim 5, wherein The specific determination of the ionic characteristic absorption wavelengths of polysulfide ions and supporting electrolytes in ultraviolet absorption includes: Preparing a polysulfide ion standard sample solution with a concentration of <10 mmol / L from the sulfur-side electrolyte solution, performing a full-spectrum scan in the wavelength range of 200 nm to 900 nm, determining the characteristic absorption wavelength and solution absorbance of the polysulfide ions, and plotting an ultraviolet absorption standard curve template with the wavelength as the abscissa and the absorbance as the ordinate.

9. The method according to claim 8, wherein The characteristic absorption wavelength of the polysulfide ions is 300 nm.

10. The method according to claim 1, characterized in that, The supporting electrolyte of the sulfur-iron redox flow battery has no characteristic absorption wavelength at 200 - 900 nm.

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