A method for preparing a high concentration polysulfide solution and its application in a flow battery

Abstract

The application relates to polysulfide technology, in particular to a high-concentration polysulfide solution preparation method and application in a liquid flow battery. The application directly reacts cheap alkali metals with sublimed sulfur to obtain a high-concentration sulfide solution. Different sulfur content solutions have corresponding standard hydrogen electrode potentials, wherein high-sulfur-content solutions such as X2S8 have the highest potential and can be used as the positive electrode of the liquid flow battery, the X2S4 with the lowest potential can be used as the negative electrode of the liquid flow battery, X=Li, Na or K, and the sulfur content of X2S8 is twice that of X2S4. The high-concentration sulfide content makes the theoretical energy density of the liquid flow battery exceed 80 Ah L ‑1 .

Description

Technical Field

[0001] This invention relates to polysulfide technology, specifically a method for preparing a high-concentration polysulfide solution and its application in flow batteries. Background Technology

[0002] Energy storage technology is crucial for the effective utilization of intermittent renewable energy sources. Grid-scale energy storage applications need to meet requirements such as high safety, long lifespan, and low cost. Redox flow batteries (ARFBs) are a promising energy storage technology with superior safety and unique chemical energy-to-electrical conversion characteristics. However, the development of ARFBs faces many challenges, including high chemical costs (all-vanadium ARFBs) and low energy density (2–15 Wh / L). −1 Organic ARFBs suffer from poor long-cycle stability (zinc-based ARFBs are plagued by severe dendrite formation); the instability of organic molecules during storage prevents ARFBs from achieving large-scale energy storage.

[0003] ARFB based on polysulfides has high solubility (approximately 3 mol / L). -1 ) and low chemical cost (￥0.9 kAh) −1 Polysulfide / bromine ARFB systems are promising for large-scale energy storage. Remick first introduced the first polysulfide / bromine ARFB system in 1984 in US patent US4485154A. However, even at low concentrations (<0.1 mol L), [the following text appears to be incomplete and requires further context: "]"] -1 In some cases, polysulfides and polyiodides also exhibit severe cross-penetration, severe membrane penetration effects, and self-discharge, leading to rapid capacity decay of ARFBs (complete failure in 50-60 cycles or 4-25 days), thus hindering the commercialization of polysulfide-based ARFBs. The rapid capacity degradation of polysulfide / polyiodide ARFBs during long cycling also impedes their development. For decades, due to the inherent low cost and high capacity of polysulfides, academia and industry have made significant efforts, and scientists are still seeking effective methods to achieve high-capacity, long-life ARFBs based on low-cost polysulfides. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing a high-concentration polysulfide solution and its application in flow batteries, resulting in an ARFB with excellent capacity and stable cycle performance.

[0005] A method for preparing a high-concentration polysulfide solution includes the following steps: Step 1, preparation of the supporting electrolyte solution: One of lithium trifluoromethanesulfonate (LiCF3SO3), lithium difluorosulfonylimide (LiFSI), lithium 2-trifluoromethanesulfonylimide (LiTFSI), sodium trifluoromethanesulfonate (NaCF3SO3), sodium difluorosulfonylimide (NaFSI), sodium 2-trifluoromethanesulfonylimide (NaTFSI), potassium trifluoromethanesulfonate (KCF3SO3), potassium difluorosulfonylimide (KFSI), and potassium 2-trifluoromethanesulfonylimide (KTFSI) is used as the supporting electrolyte; 1, One to three of the following solvents are used: dioxolane (DOL), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), diethylene glycol dimethyl ether (DME), tetraethylene glycol dimethyl ether (TETRAGLYME), and diethylene glycol dimethyl ether (DIGLYME), with a volume ratio of two or more solvents of 1:1 or 1:1:1; the supporting electrolyte is dissolved in the solvent to prepare a solution of 0.5-1.5 mol / L. -1 Supporting electrolyte solution;

[0006] Step 2, Preparation of high-concentration polysulfide solutions: In a high-purity argon glove box with H₂O ≤ 0.1 ppm and O₂ ≤ 0.1 ppm, a liquid-phase reaction method is used to directly prepare the solution. 0.5-6 mol of one alkali metal block (Li, Na, or K) is directly immersed in 1-1000 mL of the supporting electrolyte solution obtained in Step 1, while sublimed sulfur is added simultaneously. The mixture is stirred at 25-100 ℃ for 2-48 h at a stirring speed of 100-500 rpm. When 2-12 mol of sublimed sulfur is added, a high-concentration X₂S₈ polysulfide solution is obtained; when 1-6 mol of sublimed sulfur is added, a high-concentration X₂S₄ polysulfide solution is obtained; where X is Li, Na, or K.

[0007] Application of high-concentration polysulfide solutions in flow batteries: The high-concentration polysulfide solution X2S8 is used as the positive electrode of the flow battery, and the high-concentration polysulfide solution X2S4 is used as the negative electrode, where X is Li, Na, or K. The ion exchange electrolyte in the flow battery is a solid-state ion exchange electrolyte pellet. The solid-state ion exchange electrolyte pellet is prepared by a solid-phase reaction: an alkali metal source and a supporting solid-phase material are simultaneously placed in a ball mill jar for wet milling, using ethanol as the solution; the ball milling speed is 50-300 rpm, and the milling time is 10-48 h; after ball milling, vacuum drying is performed for 12-36 h to obtain solid-state ion exchange electrolyte powder; 100-500 mg of the obtained solid-state ion exchange electrolyte powder is pressed and sintered at a sintering temperature of 900-1400 ℃ to obtain a pellet with an area of ​​1-4 cm². 2Solid-state ion exchange electrolyte tablets; the alkali metal source is one of Li3PO4, Na3PO4, and K3PO4; the supporting solid phase material is 2-3 of GeO2, ZnO2, SiO2, and ZrO2, with a molar ratio of 1:1 or 1:1:1.

[0008] By placing ARFB in a battery testing cabinet and subjecting it to a constant current of 0.1-100 mA for 1-1000 cycles, the actual specific capacity of ARFB can reach 26.8-80.4 Ah L. -1 The voltage range is 0-0.3 V; the reaction products at both the positive and negative electrodes are X2S6, classifying it as a symmetrical flow battery. The solid-state ion exchange electrolyte suppresses the kink effect, preventing capacity decay. The high-concentration ARFB can cycle stably, indicating that the high concentration of sulfides, by controlling the sulfur content, achieves stable operation of the symmetrical ARFB. The presence of the solid-state ion exchange electrolyte also enhances its electrochemical stability.

[0009] This invention utilizes inexpensive alkali metals to directly react with sublimed sulfur to obtain a high-concentration sulfide solution. Solutions with different sulfur contents exhibit corresponding standard hydrogen electrode potentials, with high-sulfur solutions such as X₂S₈ (X = Li, Na, or K) possessing the highest potential and serving as the positive electrode in the flow battery, while X₂S₄, with the lowest potential, serves as the negative electrode. During discharge, the reaction product at both the positive and negative electrodes is X₂S₆, constituting a symmetrical flow battery where the electrochemical reaction involves the transfer of two electrons. The solubility of sulfides in ether-based electrolytes is as high as 3 mol / L. -1 This allows the flow battery to have a theoretical specific capacity as high as 80.4 Ah L. -1 Solid-state ion exchange electrolytes prevent sulfides from penetrating the electrode, thus avoiding capacity decay. The practical application of high-capacity ARFBs has significant practical importance and commercial value.

[0010] This invention significantly reduces the cost of ARFBs by using inexpensive alkali metals and sulfur, and these elements are abundant in the Earth's crust, ensuring no environmental pollution during future material recycling. Secondly, the prepared sulfides have extremely high concentrations, resulting in a higher theoretical specific capacity per unit volume of active material solution. Thirdly, a single reaction step is required; by controlling the molar ratio of alkali metal to sulfur, high-concentration sulfide solutions with varying sulfur content can be prepared. Fourthly, the solid-state ion exchange electrolyte avoids cross-contamination, preventing the sulfides at both ends from interpenetrating and thus avoiding capacity loss and short-circuit risks. Finally, based on the method provided by this invention, various sulfides can be applied, and the resulting sulfide solutions can be used in various ARFBs, such as halide / sulfur ARFBs, lithium-sulfur batteries, and sodium-sulfur batteries. The process equipment of this invention is simple, low-cost, and highly efficient, possessing potential for industrial application. The high-concentration sulfide electrolyte produced maintains its original high capacity while significantly improving cycle life, enabling the preparation of ARFBs with stable cycle performance up to 1000 cycles. Detailed Implementation

[0011] The embodiments of the present invention are described in detail below, but the scope of protection of the present invention is not limited to the following embodiments. The present invention uses inexpensive alkali metals to directly react with sublimed sulfur to obtain a high-concentration sulfide solution. Solutions with different sulfur contents have corresponding standard hydrogen electrode potentials. Among them, solutions with high sulfur content, such as X₂S₈ (X = Li, Na, or K), have the highest potential and can be used as the positive electrode of a flow battery, while X₂S₄ has the lowest potential and can be used as the negative electrode of a flow battery. The sulfur content in X₂S₈ is twice that in X₂S₄. The high concentration of sulfides allows the theoretical energy density of the flow battery to exceed 80 Ah / L. -1 .

[0012] Example 1: Preparation of solid-state ion-exchange electrolyte: Prepared via solid-state reaction, using Li3PO4 as the alkali metal source and GeO2 and ZrO2 as the supporting solid materials in a 1:1 ratio. The alkali metal source and supporting solid materials were simultaneously placed in a ball mill jar for wet milling with EtOH (ethanol) as the solution. The ball milling speed was 200 rpm for 24 h, followed by vacuum drying for 12 h to obtain solid-state ion-exchange electrolyte powder. 200 mg of the solid-state ion-exchange electrolyte powder was used for tablet pressing and sintering at 900 °C, yielding a tablet with an area of ​​4 cm². 2 Solid-state ion exchange electrolyte.

[0013] Preparation of the supporting electrolyte solution: The supporting electrolyte was dissolved in a solvent, with lithium trifluoromethanesulfonate (LiCF3SO3) as the supporting electrolyte and dimethyl ethylene glycol (DME) as the solvent, to prepare a 1 mol L solution. -1The supporting electrolyte solution is obtained.

[0014] Preparation of high-concentration polysulfide solutions: In a high-purity argon glove box (H₂O ≤ 0.1 ppm; O₂ ≤ 0.1 ppm), a direct liquid-phase reaction method was used. 1.0 mol of Li metal was directly immersed in 250 mL of the prepared supporting electrolyte solution, while 4.0 mol of sublimed sulfur was added. The mixture was stirred at 60 ℃ for 24 h at a stirring speed of 200 rpm to obtain a Li₂S₈ solution. Similarly, a Li₂S₄ solution could be prepared by controlling the sublimed sulfur content to 2 mol.

[0015] Application of high-concentration polysulfides in flow batteries: ARFB was subjected to constant current (10mA) cycling for 500 cycles on a battery test cabinet. Using a high-concentration Li₂S₈ solution as the positive electrode and a high-concentration Li₂S₄ solution as the negative electrode, the actual specific capacity of the ARFB reached 60.2 Ah L⁻¹. -1 With a voltage range of 0-0.3V, solid-state ion exchange electrolytes can suppress the transducer effect and avoid capacity decay. High-concentration ARFB can be stably cycled for 1000 cycles, maintaining a specific capacity of 55.6 Ah / L. -1 This indicates that high concentrations of sulfides, by controlling the sulfur content, enabled the stable operation of the symmetric ARFB. The presence of a solid ion exchange electrolyte also enhanced its electrochemical stability.

[0016] Example 2: Preparation of solid-state ion-exchange electrolyte: Prepared via solid-state reaction, using Li3PO4 as the alkali metal source and SiO2 and ZnO2 as the supporting solid materials in a 1:1 ratio. The alkali metal source and supporting solid materials were simultaneously placed in a ball mill jar for wet milling with EtOH as the solution. The milling speed was 100 rpm for 36 h. After milling, the electrolyte was vacuum dried for 24 h to obtain solid-state ion-exchange electrolyte powder. 300 mg of the powder was used for tablet pressing and sintering at 1000 °C, yielding a tablet with an area of ​​4 cm². 2 The solid-state ion exchange electrolyte is ready for use.

[0017] Preparation of the supporting electrolyte solution: The supporting electrolyte was dissolved in a solvent, with lithium bis(fluorosulfonyl)imide (LiFSI) as the supporting electrolyte; tetraethylene glycol dimethyl ether (TETRAGLYME) and tetrahydrofuran (THF) were used as solvents in a volume ratio of 1:1 to prepare a 0.8 mol / L solution. -1 The supporting electrolyte solution is obtained and left to stand for later use.

[0018] Preparation of high-concentration polysulfide solutions: In a glove box filled with high-purity argon (H₂O ≤ 0.1 ppm; O₂ ≤ 0.1 ppm), a direct liquid-phase reaction method was used. 2 mol of Li metal blocks were directly immersed in 1000 mL of the prepared supporting electrolyte solution, while 8 mol of sublimed sulfur was added. The mixture was stirred at 40 ℃ for 36 h at a stirring speed of 100 rpm to obtain a Li₂S₈ solution, which was then allowed to stand for later use. Similarly, by controlling the sublimed sulfur content to 4 mol, a Li₂S₄ solution could be prepared and allowed to stand for later use.

[0019] Application of high-concentration polysulfides in flow batteries: ARFBs were subjected to constant current (100mA) cycling for 1000 cycles on a battery testing cabinet. Using a high-concentration Li₂S₈ solution as the positive electrode and a high-concentration Li₂S₄ solution as the negative electrode, the actual specific capacity of the ARFB reached 45.6 Ah L. -1 With a voltage range of 0-0.28 V, solid-state ion exchange electrolytes can suppress the transducer effect and avoid capacity decay. High-concentration ARFB can be stably cycled for 1000 cycles, maintaining a specific capacity of 40.2 Ah / L. -1 This indicates that high concentrations of sulfides, by controlling the sulfur content, enabled the stable operation of the symmetric ARFB. The presence of a solid ion exchange electrolyte also enhanced its electrochemical stability.

[0020] Example 3: Preparation of solid-state ion-exchange electrolyte: Prepared via solid-state reaction, using Na3PO4 as the alkali metal source and ZnO2, SiO2, and ZrO2 as the supporting solid-state material in a 1:1:1 ratio. The alkali metal source and supporting solid-state material were simultaneously placed in a ball mill jar for wet milling with EtOH as the solution. The milling speed was 300 rpm for 16 h. After milling, the mixture was vacuum dried for 15 h to obtain solid-state ion-exchange electrolyte powder. 250 mg of the solid-state ion-exchange electrolyte powder was used for tablet pressing and sintering at 1000 °C, yielding a tablet with an area of ​​4 cm². 2 The solid-state ion exchange electrolyte is ready for use.

[0021] Preparation of the supporting electrolyte solution: The supporting electrolyte was dissolved in a solvent, using sodium bis(fluorosulfonyl)imide (NaFSI) as the supporting electrolyte and diethylene glycol dimethyl ether (DIGLYME) as the solvent, to prepare a 0.9 mol / L solution. -1 The supporting electrolyte solution is obtained and left to stand for later use.

[0022] Preparation of high-concentration polysulfide solutions: In a glove box filled with high-purity argon (H₂O ≤ 0.1 ppm; O₂ ≤ 0.1 ppm), a direct liquid-phase reaction method was used. 0.5 mol of Na metal was directly immersed in 250 mL of the prepared supporting electrolyte solution, while 2 mol of sublimed sulfur was added. The mixture was stirred at 60 ℃ for 36 h at a stirring speed of 300 rpm to obtain a Na₂S₈ solution, which was then allowed to stand for later use. Similarly, by controlling the sublimed sulfur content to 1 mol, a Na₂S₄ solution could be prepared and allowed to stand for later use.

[0023] Application of high-concentration polysulfides in flow batteries: ARFBs were subjected to constant current (100mA) cycling for 1000 cycles on a battery testing cabinet. Using a high-concentration Na₂S₈ solution as the positive electrode and a high-concentration Na₂S₄ solution as the negative electrode, the actual specific capacity of the ARFB reached 48.6 Ah L. -1 With a voltage range of 0-0.22 V, solid-state ion exchange electrolytes can suppress the transducer effect and avoid capacity decay. High-concentration ARFB can be stably cycled for 1000 cycles, maintaining a specific capacity of 45.0 Ah / L. -1 This indicates that high concentrations of sulfides, by controlling the sulfur content, enabled the stable operation of the symmetric ARFB. The presence of a solid ion exchange electrolyte also enhanced its electrochemical stability.

[0024] Example 4: Preparation of solid-state ion-exchange electrolyte: Prepared via solid-state reaction, using Na3PO4 as the alkali metal source and ZnO2 and GeO2 as the supporting solid-state materials in a 1:1 ratio. The alkali metal source and supporting solid-state materials were simultaneously placed in a ball mill jar for wet milling with EtOH as the solution. The milling speed was 280 rpm for 18 h. After milling, the electrolyte was vacuum dried for 16 h to obtain solid-state ion-exchange electrolyte powder. 180 mg of the solid-state ion-exchange electrolyte powder was used for tablet pressing and sintering at 950 ℃, yielding a tablet with an area of ​​4 cm². 2 The solid-state ion exchange electrolyte is ready for use.

[0025] Preparation of the supporting electrolyte solution: The supporting electrolyte was dissolved in a solvent, with lithium 2-trifluoromethanesulfonylimide (LiTFSI) as the supporting electrolyte and diethylene glycol dimethyl ether (DIGLYME) as the solvent, to prepare a 1.2 mol L solution. -1 The supporting electrolyte solution is obtained and left to stand for later use.

[0026] Preparation of high-concentration polysulfide solutions: In a glove box filled with high-purity argon (H₂O ≤ 0.1 ppm; O₂ ≤ 0.1 ppm), a direct liquid-phase reaction method was used. A 3 mol molar mass of Na metal block was directly immersed in 1000 mL of the prepared supporting electrolyte solution, while 12 mol of sublimed sulfur was added. The mixture was stirred at 60 ℃ for 36 h at a stirring speed of 300 rpm to obtain a Na₂S₈ solution, which was then allowed to stand for later use. Similarly, a Na₂S₄ solution could be prepared by controlling the sublimed sulfur content to 6 mol and then allowed to stand for later use.

[0027] Application of high-concentration polysulfides in flow batteries: ARFBs were subjected to constant current (200mA) cycling for 1000 cycles on a battery testing cabinet. Using a high-concentration Na₂S₈ solution as the positive electrode and a high-concentration Na₂S₄ solution as the negative electrode, the actual specific capacity of the ARFB reached 70.6 Ah L. -1 With a voltage range of 0-0.22V, solid-state ion exchange electrolytes can suppress the transducer effect and avoid capacity decay. High-concentration ARFB can be stably cycled for 1000 cycles, maintaining a specific capacity of 65.8 Ah / L. -1 This indicates that high concentrations of sulfides, by controlling the sulfur content, enabled the stable operation of the symmetric ARFB. The presence of a solid ion exchange electrolyte also enhanced its electrochemical stability.

[0028] Example 5: Preparation of solid-state ion-exchange electrolyte: Prepared via solid-state reaction, using K3PO4 as the alkali metal source and SiO2 and ZrO2 as the supporting solid-state materials in a 1:1 ratio. The alkali metal source and supporting solid-state materials were simultaneously placed in a ball mill jar for wet milling with EtOH as the solution. The milling speed was 250 rpm for 20 h. After milling, the mixture was vacuum dried for 24 h to obtain solid-state ion-exchange electrolyte powder. 300 mg of the solid-state ion-exchange electrolyte powder was used for tablet pressing and sintering at 1100 ℃, yielding a tablet with an area of ​​4 cm². 2 The solid-state ion exchange electrolyte is ready for use.

[0029] Preparation of the supporting electrolyte solution: The supporting electrolyte was dissolved in a solvent. Potassium trifluoromethanesulfonate (KCF3SO3) was used as the supporting electrolyte, and tetraethylene glycol dimethyl ether (TETRAGLYME), diethylene glycol dimethyl ether (DIGLYME), and tetrahydrofuran (THF) were used as solvents in a volume ratio of 1:1:1 to prepare a 1 mol L solution. -1 The supporting electrolyte solution is obtained and left to stand for later use.

[0030] Preparation of high-concentration polysulfide solutions: In a glove box filled with high-purity argon (H₂O ≤ 0.1 ppm; O₂ ≤ 0.1 ppm), solutions were prepared directly via a liquid-phase reaction. 3 mol of potassium metal was directly immersed in 1000 mL of the prepared supporting electrolyte solution, while 12 mol of sublimed sulfur was added. The mixture was stirred at 60 ℃ for 24 h at a stirring speed of 200 rpm, yielding a K₂S₈ solution which was then allowed to stand for later use. Similarly, a K₂S₄ solution could be prepared by controlling the sublimed sulfur content to 6 mol and then allowing it to stand for later use.

[0031] Application of high-concentration polysulfides in flow batteries: ARFB was subjected to constant current (200mA) cycling for 1000 cycles on a battery test cabinet. Using a high-concentration K₂S₈ solution as the positive electrode and a high-concentration K₂S₄ solution as the negative electrode, the actual specific capacity of the ARFB reached 68.9 Ah L. -1 With a voltage range of 0-0.18 V, solid-state ion exchange electrolytes can suppress the transducer effect and avoid capacity decay. High-concentration ARFB can be stably cycled for 1000 cycles, maintaining a specific capacity of 65.6 Ah / L. -1 This indicates that high concentrations of sulfides, by controlling the sulfur content, enabled the stable operation of the symmetric ARFB. The presence of a solid ion exchange electrolyte also enhanced its electrochemical stability.

Claims

1. A flow battery, characterized in that: A high-concentration polysulfide solution of X₂S₈ was used as the positive electrode of the flow battery, and a high-concentration polysulfide solution of X₂S₄ was used as the negative electrode, where X is Li, Na, or K. The ion exchange electrolyte in the flow battery was a solid-state ion exchange electrolyte pellet. The solid-state ion exchange electrolyte pellet was prepared via a solid-phase reaction: an alkali metal source and a supporting solid-phase material were simultaneously placed in a ball mill jar for wet milling in ethanol; the milling speed was 50-300 rpm, and the milling time was 10-48 h; after milling, the pellet was vacuum dried for 12-36 h to obtain solid-state ion exchange electrolyte powder; 100-500 mg of the obtained solid-state ion exchange electrolyte powder was pressed and sintered at a temperature of 900-1400 ℃ to obtain a pellet with an area of ​​1-4 cm². 2 Solid-state ion exchange electrolyte tablets; the alkali metal source is one of Li3PO4, Na3PO4, and K3PO4; the supporting solid phase material is 2-3 of GeO2, ZnO2, SiO2, and ZrO2, with a molar ratio of 1:1 or 1:1:

1. The preparation method of a high-concentration polysulfide solution includes the following steps: Step 1, preparation of the supporting electrolyte solution: One of lithium trifluoromethanesulfonate, lithium difluorosulfonylimide, sodium trifluoromethanesulfonate, sodium difluorosulfonylimide, potassium trifluoromethanesulfonate, and potassium difluorosulfonylimide is used as the supporting electrolyte; one to three of 1,3-dioxolane, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and diethylene glycol dimethyl ether are used as solvents, with a volume ratio of two or more solvents of 1:1 or 1:1:1; the supporting electrolyte is dissolved in the solvent to prepare a solution of 0.5-1.5 mol·L⁻¹. -1 Supporting electrolyte solution; Step 2, Preparation of high-concentration polysulfide solutions: In a high-purity argon glove box with H₂O ≤ 0.1 ppm and O₂ ≤ 0.1 ppm, a liquid-phase reaction method is used to directly prepare the solution. 0.5-6 mol of one alkali metal block (Li, Na, or K) is directly immersed in 1-1000 mL of the supporting electrolyte solution obtained in Step 1, while sublimed sulfur is added simultaneously. The mixture is stirred at 25-100 ℃ for 2-48 h at a stirring speed of 100-500 rpm. When 2-12 mol of sublimed sulfur is added, a high-concentration X₂S₈ polysulfide solution is obtained; when 1-6 mol of sublimed sulfur is added, a high-concentration X₂S₄ polysulfide solution is obtained; where X is Li, Na, or K.

2. The flow battery according to claim 1, characterized in that: When the flow batteries were placed in a battery testing cabinet and subjected to a constant current of 0.1-100 mA and cyclic testing of 1-1000 cycles, the actual specific capacity of the flow batteries reached 26.8-80.4 Ah·L. -1 The voltage range is 0-0.3 V; the reaction products at both the positive and negative electrodes are X2S6, which is a symmetrical flow battery.