High energy density electrolyte

Abstract

A system and method are provided for an electrolyte for a flow battery comprising a redox active species and a plurality of supporting salts dissolved in the electrolyte, the redox active species having a concentration greater than 2.0 M, and the plurality of dissolved supporting salts comprising a potassium salt, an ammonium salt, a calcium salt, and a manganese salt.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 369,728, entitled "HIGH ENERGY DENSITY ELECTROLYTE," filed July 28, 2022. The entire contents of the above-identified application are incorporated herein by reference for all purposes.

[0002] The present specification relates generally to electrolytes for redox flow batteries. [Background technology]

[0003] Redox flow batteries are well-suited for grid-scale energy storage applications due to their ability to scale power and capacity independently, and for charging and discharging over thousands of cycles with little performance loss compared to conventional battery technologies. Iron hybrid redox flow batteries are particularly attractive due to the low-cost materials incorporated into the cell stack. Iron redox flow batteries (IFBs) rely on iron, salt, and water as the electrolyte. One way to increase the capacity of an IFB without increasing its size (e.g., the size of the electrolyte tank) is to increase the concentration of iron salt dissolved in the electrolyte. Summary of the Invention [Problem to be solved by the invention]

[0004] Attempts to increase the iron salt concentration in the electrolyte have been limited by the inherent solubility of different salts contained in IFB electrolytes. Iron salts can be used in electrolytes at concentrations of 1.7 M as part of conventional IFB electrolyte solutions. Alternatively, concentrations as high as 5.5 M have been reported in the field of iron electrodeposition. However, electrolytes used for iron electrodeposition, or any other electrolytes that do not contain sufficient electrochemically inert supporting salts, may not be easily transferred to IFB systems without undesirable reductions in coulombic efficiency. Additionally, electrodeposition electrolytes may be formulated for use at high temperatures (e.g., above 60°C) and therefore may be incompatible with auxiliary components of the piping used in IFBs. [Means for solving the problem]

[0005] The present inventors have recognized the above-mentioned shortcomings of previous strategies for increasing the iron salt concentration in an IFB electrolyte and have developed an electrolyte composition that at least partially overcomes these shortcomings. In one example, an electrolyte for a redox flow battery may include a redox-active species dissolved in the electrolyte and having a concentration of at least 2.0 M, in addition to multiple dissolved supporting salts, including potassium, ammonium, calcium, and manganese salts. The electrolyte composition may enable an increase in the capacity of the IFB without increasing the size of the electrolyte tank or sacrificing battery stability. The electrolyte composition can be used with existing IFB components (e.g., cell stack, piping, pumps, etc.), allowing for economical and efficient replacement of conventional electrolytes with the electrolyte composition described herein. Furthermore, the electrolyte may remain an aqueous electrolyte, and the composition may maintain the low cost and low toxicity characteristics valued in IFB electrolytes.

[0006] It should be understood that the foregoing Summary is provided to introduce a selection of concepts in a simplified form that are further described in the Detailed Description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the Detailed Description. Moreover, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or anywhere in this disclosure. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 shows a schematic diagram of an exemplary redox flow battery system including an electrolyte. [Figure 2] 1 shows a plot of the percentage of theoretical discharge capacity as a function of the number of charge cycles for IFBs comparing results from a reference electrolyte, a first high energy density electrolyte, and a second high energy density electrolyte. [Figure 3] 1 shows a plot of potential as a function of time for a reference electrolyte. [Figure 4] 1 shows a plot of potential as a function of time for a first high energy density electrolyte. [Figure 5] 1 shows a plot of potential as a function of time for a second high energy density electrolyte. [Figure 6] 1 shows a plot of efficiency as a function of the number of charge cycles for a first high energy density electrolyte. [Figure 7] 10 shows a plot of efficiency as a function of the number of charge cycles for a second high energy density electrolyte. [Figure 8] 1 shows a plot of the percentage of theoretical discharge capacity as a function of the number of charge cycles in an IFB comparing the results for a first high energy density electrolyte with a second high energy density electrolyte. [Figure 9] 1 shows a plot comparing voltage profiles over time for a first high energy density electrolyte and a second high energy density electrolyte. DETAILED DESCRIPTION OF THE INVENTION

[0008] The following description relates to the composition of a high-energy-density electrolyte for an IFB. The electrolyte can be used in an IFB as shown schematically in Figure 1. The energy density of an IFB electrolyte can be increased by increasing the concentration of dissolved iron species in the electrolyte. The increased concentration of iron salt in the high-energy-density electrolyte described herein can be achieved without sacrificing IFB performance by adjusting the supporting salt in the electrolyte composition. As used herein, a supporting salt may refer to a salt contained in an electrolyte for a redox flow battery that does not participate in a redox reaction. For example, in a first example of a high-energy-density electrolyte, ammonium chloride may be added to the baseline electrolyte. In a second example of a high-energy-density electrolyte, ammonium chloride and calcium chloride may be added. The baseline electrolyte may include potassium chloride, manganese chloride, and boric acid in addition to the iron salt. Figure 2 shows a plot comparing the percentage of theoretical discharge capacity over multiple cycles for an IFB containing a baseline electrolyte (as a reference electrolyte) and the first and second embodiments of the high-energy-density electrolyte. Figures 3-5 further compare the electrolytes listed above by showing plots of potential as a function of time for each electrolyte. Figures 6-7 further characterize the first and second examples of high-energy-density electrolytes by showing plots comparing efficiency as a function of the number of charge cycles for each of the electrolytes. The performance of the first embodiment of the high-energy-density electrolyte and the second embodiment of the high-energy-density electrolyte is highlighted in Figure 8. For example, Figure 8 further shows a plot of the data shown in Figure 2, but does not include data from the baseline electrolyte. Figure 9 plots an overlay of voltage profiles over time.

[0009] 1, in the redox flow battery system 10, the anode 26 may be referred to as the plating electrode and the cathode 28 may be referred to as the redox electrode. The negative electrolyte in the plating side (e.g., anode compartment 20) of the redox flow battery cell 18 may be referred to as the plating electrolyte, and the positive electrolyte on the redox side (e.g., cathode compartment 22) of the redox flow battery cell 18 may be referred to as the redox electrolyte.

[0010] "Anode" refers to the electrode where the electroactive material loses electrons, and "cathode" refers to the electrode where the electroactive material gains electrons. During battery charging, the negative electrolyte gains electrons at the negative electrode 26, which is the cathode of the electrochemical reaction. During battery discharge, the negative electrolyte loses electrons, and the negative electrode 26 is the anode of the electrochemical reaction. Alternatively, during battery discharge, the negative electrolyte and negative electrode 26 may be referred to as the anolyte and anode of the electrochemical reaction, respectively, while the positive electrolyte and positive electrode 28 may be referred to as the catholyte and cathode of the electrochemical reaction, respectively. During battery charging, the negative electrolyte and negative electrode 26 may be referred to as the catholyte and cathode of the electrochemical reaction, respectively, while the positive electrolyte and positive electrode 28 may be referred to as the anolyte and anode of the electrochemical reaction, respectively. For brevity, the terms "positive" and "negative" are used herein to refer to the electrodes, electrolytes, and electrode compartments in a redox flow battery system.

[0011] One example of a hybrid redox flow battery is an all-iron redox flow battery (IFB), in which the electrolyte includes iron ions in the form of iron salts (e.g., FeCl, FeCl, etc.) and the anode 26 includes metallic iron. For example, in the anode 26, ferrous iron (Fe 2+ ) gains two electrons during battery charging and becomes iron metal (Fe 0 ) was plated on the negative electrode 26 as Fe 0 loses two electrons during battery discharge, becoming Fe 2+ In the positive electrode 28, Fe 2+ During battery charging, ferric iron (Fe 3+ ) and Fe 3+During battery discharge, Fe 2+ The electrochemical reactions are summarized in equations (1) and (2), where the forward reaction (left to right) represents the electrochemical reaction during battery charging, while the reverse reaction (right to left) represents the electrochemical reaction during battery discharging. (chemical 1) Fe 2+ +2e - <―>Fe 0 -0.44V (negative pole) (1) (chemical 2) 2Fe 2+ <―>2Fe 3+ +2e - +0.77V (positive) (2)

[0012] As mentioned above, the negative electrolyte used in the IFB is a volatile electrolyte that dissolves Fe during battery charging. 2- accepts two electrons from the negative electrode 26 and 0 and plated onto the substrate. 2+ During battery discharge, the plated Fe 0 loses two electrons to form Fe 2+ The equilibrium potential of the above reaction is -0.44 V, so this reaction provides the negative terminal for the desired system. On the positive side of the IFB, the electrolyte loses electrons and dissolves back into the electrolyte during battery charging. 3+ Fe oxidizes to 2- During battery discharge, Fe provided by the electrolyte 3- by absorbing electrons provided by the positive electrode 28. 2+ The equilibrium potential for this reaction is +0.77 V, creating a positive terminal in the desired system.

[0013] IFBs may offer the ability to charge and recharge the electrolyte therein, as opposed to other battery types that utilize non-regenerating electrolytes. Charging may be accomplished by applying current across electrodes 26 and 28 via terminals 40 and 42, respectively. Anode 26 is charged with Fe in a positive electrolyte in cathode compartment 22 (e.g., Fe 2+ But Fe 3+The negative electrode 26 may be electrically coupled to the negative side of a voltage source via terminal 40 so that electrons can be delivered to the negative electrolyte via the positive electrode 28 (when the negative electrode 26 is oxidized to Fe 2+ is reduced to form Fe on the (plating) substrate. 0 can form Fe 2+ is plated onto the negative electrode 26.

[0014] The discharge is Fe 0 remains available in the negative electrolyte for oxidation, and Fe 3+ As an example, Fe can persist while Fe remains available in the positive electrolyte for reduction. 3+ The availability of Fe can be increased by adding additional Fe via an external source such as an external positive electrolyte chamber 52. 3+ This can be maintained by increasing the concentration or volume of the positive electrolyte in the positive electrode compartment 22 of the redox flow battery cell 18 to provide ions. More generally, Fe 0 The availability of Fe can be an issue in IFB systems, where the Fe available for discharge 0 can be proportional to the surface area and volume of the negative electrode substrate and the plating efficiency. 2+ As an example, Fe 2+ The availability of Fe can be increased by adding additional Fe via an external source, such as the external negative electrolyte chamber 50. 2+ The negative electrolyte concentration or volume can be maintained by providing ions to the negative electrode compartment 20 of the redox flow battery cell 18. It can be appreciated that increasing the iron concentration in the positive and negative electrolytes can increase the capacity of the IFB system without increasing the volume of the electrolyte. In this manner, the energy density of the IFB system can be increased.

[0015] In an IFB, the positive electrolyte may include ferrous iron, ferric iron, a ferric iron complex, or any combination thereof, while the negative electrolyte may include ferrous iron or a ferrous iron complex, depending on the state of charge (SOC) of the IFB system. As previously described, the use of iron ions in both the negative and positive electrolytes may allow for the use of the same electrolyte species on both sides of the redox flow battery cell 18, which may reduce electrolyte cross-contamination and increase the efficiency of the IFB system, resulting in less electrolyte exchange compared to other redox flow battery systems. Adding a supporting salt to the electrolyte, as described below, may allow for an increased iron concentration in the electrolyte solution. The supporting salt may be a salt that increases the conductivity of the electrolyte solution and also aids in the stability of the redox-active salt (e.g., FeCl), but is not oxidized or reduced during operation of the redox flow battery.

[0016] Efficiency losses in IFBs can result from electrolyte crossover through the separator 24 (e.g., ion exchange membrane barrier, microporous membrane, etc.). For example, Fe in the positive electrolyte 3+ The ion is Fe 3+ The Fe ions can be driven towards the negative electrolyte by the ion concentration gradient and electrophoretic forces across the separator 24. The Fe ions then penetrate the separator 24 and cross over into the negative electrode compartment 20. 3+ ions can result in coulombic efficiency loss. Fe ions crossing over from the low pH redox side (e.g., the more acidic cathode compartment 22) to the high pH plating side (e.g., the less acidic anode compartment 20) 3+The ions can lead to the precipitation of Fe(OH)3. The precipitation of Fe(OH)3 can degrade the separator 24 and cause permanent battery performance and efficiency loss. For example, Fe(OH)3 precipitates can chemically foul the organic functional groups of the ion-exchange membrane or physically clog the pores of the ion-exchange membrane. In either case, the Fe(OH)3 precipitates can increase the membrane's ohmic resistance over time and reduce battery performance. While the precipitates can be removed by washing the IFB with acid, the constant maintenance and downtime can be disadvantageous for commercial battery applications. Furthermore, washing can depend on periodic electrolyte preparation, contributing to additional processing costs and complexity. Alternatively, adding specific organic acids to the positive and negative electrolytes in response to electrolyte pH changes can mitigate precipitate formation during battery charge and discharge cycling without increasing overall costs. Additionally, Fe 3+ Implementing a membrane barrier that inhibits ionic crossover may also mitigate fouling.

[0017] Additional coulombic efficiency loss occurs due to H + (e.g., protons) and subsequent formation of H2 gas, and the reaction of protons in the anode compartment 20 with electrons supplied to the plated iron metal of the anode 26, can form H2 gas.

[0018] IFB electrolytes (e.g., FeCl2, FeCl3, FeSO4, Fe2(SO4)3, etc.) may be readily available and can be produced at low cost. In one example, an IFB electrolyte may be formed from ferrous chloride (FeCl2), potassium chloride (KCl), manganese(II) chloride (MnCl2), and boric acid (H3BO3). Additionally, ammonium chloride (NH4Cl) and calcium chloride (CaCl2) may be included in the electrolyte to increase the Fe concentration in solution. 2+ / 3+The IFB electrolyte may allow the concentration of iron to be increased beyond that typically considered stable. In this way, a high-energy density electrolyte may be formed. Because the same electrolyte can be used for the negative and positive electrolytes, the IFB electrolyte may offer higher recycling values, resulting in reduced cross-contamination issues compared to other systems. Furthermore, due to the electron configuration of iron, iron may solidify into a generally uniform solid structure during plating onto the negative electrode substrate. In the case of zinc and other metals commonly used in hybrid redox batteries, solid dendritic structures may form during plating. The stable electrode morphology of the IFB system may increase the efficiency of the battery compared to other redox flow batteries. Furthermore, the iron redox flow battery may reduce the use of toxic raw materials and operate at a relatively neutral pH compared to other redox flow battery electrolytes. Therefore, the IFB system may reduce environmental hazards compared to all other current advanced redox flow battery systems in production.

[0019] Continuing with FIG. 1 , a schematic diagram of a redox flow battery system 10 is shown. The redox flow battery system 10 may include a redox flow battery cell 18 fluidly coupled to an integrated multi-chamber electrolyte storage tank 110. The redox flow battery cell 18 may include an anode compartment 20, a separator 24, and a cathode compartment 22. The separator 24 may include an electrically insulating ionically conductive barrier that prevents bulk mixing of the positive and negative electrolytes while allowing conduction of certain ions. For example, as described above, the separator 24 may include an ion exchange membrane and / or a microporous membrane.

[0020] The anode compartment 20 may include an anode 26, and the negative electrolyte may include an electroactive material. The cathode compartment 22 may include a cathode 28, and the positive electrolyte may include an electroactive material. In some embodiments, multiple redox flow battery cells 18 may be combined in series or parallel to produce higher voltages or currents in the redox flow battery system 10.

[0021] 1 are negative and positive electrolyte pumps 30 and 32, both of which are used to pump the electrolyte solution through the redox flow battery system 10. The electrolyte is stored in one or more tanks external to the cell and is pumped through the anode compartment 20 and cathode compartment 22 sides of the redox flow battery cell 18 via the negative and positive electrolyte pumps 30 and 32, respectively.

[0022] The redox flow battery system 10 may also include a first bipolar plate 36 and a second bipolar plate 38, each positioned along the back side of the negative electrode 26 and positive electrode 28, respectively, e.g., opposite the side facing the separator 24. The first bipolar plate 36 may be in contact with the negative electrode 26, and the second bipolar plate 38 may be in contact with the positive electrode 28. However, in other embodiments, the bipolar plates 36 and 38 may be arranged in close proximity to the electrodes 26 and 28 but spaced apart from the electrodes 26 and 28 and housed within the respective electrode compartments 20 and 22. In either case, the bipolar plates 36 and 38 may be electrically coupled to terminals 40 and 42, respectively, via direct contact with or via the negative and positive electrodes 26 and 28, respectively. The IFB electrolyte may be transported to the reaction sites on the negative and positive electrodes 26 and 28 by the first and second bipolar plates 36 and 38 due to the conductive properties of the materials of the bipolar plates 36 and 38. The flow of electrolyte may also be assisted by the negative and positive electrolyte pumps 30 and 32, which facilitate forced convection through the redox flow battery cell 18. Reacted electrochemical species may also be directed away from the reaction sites by a combination of forced convection and the presence of the first and second bipolar plates 36 and 38.

[0023] As shown in FIG. 1 , the redox flow battery cell 18 may further include a negative battery terminal 40 and a positive battery terminal 42. When a charging current is applied to the battery terminals 40 and 42, the positive electrolyte may be oxidized (lose one or more electrons) at the positive electrode 28, and the negative electrolyte may be reduced (gain one or more electrons) at the negative electrode 26. During battery discharge, reverse redox reactions may occur at the electrodes 26 and 28. In other words, the positive electrolyte may be reduced (gain one or more electrons) at the positive electrode 28, and the negative electrolyte may be oxidized (lose one or more electrons) at the negative electrode 26. A potential difference across the battery may be maintained by electrochemical redox reactions in the positive electrode compartment 22 and the negative electrode compartment 20, which may induce a current through the current collectors while the reactions persist. The amount of energy stored by a redox battery may be limited by the amount of electroactive material available in the electrolyte for discharge, depending on the total amount of electrolyte and the solubility of the electroactive material.

[0024] The redox flow battery system 10 may further include an integrated multi-chamber electrolyte storage tank 110. The multi-chamber electrolyte storage tank 110 may be divided by a bulkhead 98. The bulkhead 98 may create multiple chambers within the multi-chamber electrolyte storage tank 110 so that both positive and negative electrolytes may be contained within a single tank. The negative electrolyte chamber 50 holds a negative electrolyte containing an electroactive material, and the positive electrolyte chamber 52 holds a positive electrolyte containing an electroactive material. The bulkhead 98 may be positioned within the multi-chamber electrolyte storage tank 110 to provide a desired volume ratio between the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In one embodiment, the bulkhead 98 may be positioned to set the volume ratio of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 according to the stoichiometric ratio between the negative and positive redox reactions. FIG. 1 also shows a fill height 112 of the multi-chamber electrolyte storage tank 110, which may indicate the liquid level within each tank compartment. 1 also shows a gas headspace 90 located above the fill height 112 of the negative electrolyte chamber 50 and a gas headspace 92 located above the fill height 112 of the positive electrolyte chamber 52. The gas headspace 92 can be utilized to store H gas generated during operation of the redox flow battery (e.g., by the side reactions of proton reduction and iron corrosion) and carried to the multi-chamber electrolyte storage tank 110 with the electrolyte returning from the redox flow battery cell 18. The H gas can naturally separate at the gas-liquid interface (e.g., fill height 112) within the multi-chamber electrolyte storage tank 110, thereby eliminating the need for an additional gas-liquid separator as part of the redox flow battery system 10. Once separated from the electrolyte, the H gas can fill the gas headspaces 90 and 92. In this way, the stored H gas can help purge other gases from the multi-chamber electrolyte storage tank 110, thereby acting as an inert gas blanket to reduce oxidation of electrolyte species, which can help reduce redox flow battery capacity loss.In this manner, utilizing the integrated multi-chamber electrolyte storage tank 110 may eliminate the need for separate negative and positive electrolyte storage tanks, hydrogen storage tanks, and gas-liquid separators common to conventional redox flow battery systems, thereby simplifying system design, reducing the physical footprint of the redox flow battery system 10, and reducing system costs.

[0025] 1 also shows a spillover hole 96, which may create an opening in a bulkhead 98 between the gas headspaces 90 and 92 and provide a means for equalizing gas pressure between the chambers 50 and 52. The spillover hole 96 may be positioned at a threshold height above the fill height 112. The spillover hole 96 may further enable the ability to self-balance the electrolyte in each of the negative and positive electrolyte chambers 50 and 52 in the event of a cell crossover. In the case of an all-iron redox flow battery system, the same electrolyte (Fe 2+ ) may be used in both the negative and positive electrode compartments 20 and 22, so that although electrolyte spillover between the negative electrolyte chamber 50 and the positive electrolyte chamber 52 may reduce overall system efficiency, overall electrolyte composition, battery module performance, and battery module capacity may be maintained. Flange joints may be utilized for all piping connections between the inlets and outlets to the multi-chamber electrolyte storage tank 110 to maintain a leak-free, continuously pressurized state. The multi-chamber electrolyte storage tank 110 may include at least one outlet from each of the negative and positive electrolyte chambers 50 and 52 and at least one inlet to each of the negative and positive electrolyte chambers 50 and 52. Additionally, one or more outlet connections may be provided from the gas headspaces 90 and 92 to direct H gas to the rebalancing reactors or cells 80 and 82.

[0026] Although not shown in FIG. 1 , the integrated multi-chamber electrolyte storage tank 110 may further include one or more heaters thermally coupled to each of the negative and positive electrolyte chambers 50 and 52. In an alternative embodiment, only one of the negative and positive electrolyte chambers 50 and 52 may include one or more heaters. If only the positive electrolyte chamber 52 includes one or more heaters, the negative electrolyte may be heated by transferring heat generated in the redox flow battery cell 18 to the negative electrolyte. In this manner, the redox flow battery cell 18 may heat the negative electrolyte and facilitate temperature regulation of the negative electrolyte. The one or more heaters may be operated by the controller 88 to regulate the temperatures of the negative and positive electrolyte chambers 50 and 52 independently or together. For example, in response to the electrolyte temperature falling below a threshold temperature, the controller 88 may increase the power supplied to the one or more heaters so that heat flux to the electrolyte may increase. The electrolyte temperature may be indicated by one or more temperature sensors attached to the multi-chamber electrolyte storage tank 110, such as sensors 60 and 62. By way of example, the one or more heaters may include coil-type heaters or other immersion heaters immersed in the electrolyte fluid, or surface mantle-type heaters that conductively transfer heat through the walls of the negative and positive electrolyte chambers 50 and 52 to heat the fluid therein. Other known types of tank heaters may be used without departing from the scope of this disclosure. Additionally, the controller 88 may deactivate one or more heaters in the negative and positive electrolyte chambers 50 and 52 in response to a liquid level below a solids fill threshold level. Alternatively, in some embodiments, the controller 88 may activate one or more heaters in the negative and positive electrolyte chambers 50 and 52 only in response to a liquid level above a solids fill threshold level. In this manner, activation of one or more heaters without sufficient liquid in the negative and / or positive electrolyte chambers 50, 52 may be avoided, thereby reducing the risk of overheating or burning out of the heater(s).

[0027] Furthermore, one or more inlet connections may be provided to each of the negative and positive electrolyte chambers 50 and 52 from a field hydration system (not shown). In this manner, the field hydration system may facilitate start-up of the redox flow battery system 10 at the end-use site, including installation, filling, and hydration of the redox flow battery system 10. Furthermore, prior to start-up of the redox flow battery system 10 at the end-use site, the redox flow battery system 10 may be dry-assembled at a battery manufacturing facility different from the end-use site without having to fill and hydrate the redox flow battery system 10 before shipping it to the end-use site. In one embodiment, the end-use site may correspond to a location where the redox flow battery system 10 will be installed and utilized for on-site energy storage. In other words, the redox flow battery system 10 may be designed such that once installed and hydrated at the end-use site, the location of the redox flow battery system 10 is fixed and the redox flow battery system 10 can no longer be considered a portable dry system. Thus, from an end user's perspective, the dry, portable redox flow battery system 10 may be delivered to a site, after which the redox flow battery system 10 may be installed, hydrated, and commissioned. Prior to hydration, the redox flow battery system 10 may be referred to as a dry, portable system, in which the redox flow battery system 10 is free of or does not include water and wet electrolyte. Once hydrated, the redox flow battery system 10 may be referred to as a wet, non-portable system, in which the redox flow battery system 10 includes wet electrolyte.

[0028] 1 , the electrolyte solution initially stored in the multi-chamber electrolyte storage tank 110 can be pumped throughout the redox flow battery system 10 via negative and positive electrolyte pumps 30 and 32. The electrolyte stored in the negative electrolyte chamber 50 can be pumped through the negative compartment 20 side of the redox flow battery cell 18 via the negative electrolyte pump 30, and the electrolyte stored in the positive electrolyte chamber 52 can be pumped through the positive compartment 22 side of the redox flow battery cell 18 via the positive electrolyte pump 32.

[0029] The electrolyte rebalancing reactors 80 and 82 may be connected in-line or in parallel with the electrolyte recirculation flow paths at the negative and positive sides, respectively, of the redox flow battery cells 18 in the redox flow battery system 10. One or more rebalancing reactors may be connected in-line with the electrolyte recirculation flow paths at the negative and positive sides of the battery, and other rebalancing reactors may be connected in parallel for redundancy (e.g., a rebalancing reactor may be maintained without interrupting the battery and rebalancing operation) and increased rebalancing capacity. In one example, the electrolyte rebalancing reactors 80 and 82 may be disposed in the return flow paths from the negative and positive electrode compartments 20 and 22 to the negative and positive electrolyte chambers 50 and 52, respectively.

[0030] The electrolyte rebalancing reactors 80 and 82 can serve to rebalance electrolyte charge imbalances in the redox flow battery system 10 that occur due to side reactions, ionic crossover, etc., as described herein. In one embodiment, the electrolyte rebalancing reactors 80 and 82 can include trickle-bed reactors, where H gas and electrolyte can contact at a catalyst surface within a packed bed to perform the electrolyte rebalancing reaction. In other embodiments, the rebalancing reactors 80 and 82 can include flow-through reactors that can contact H gas and electrolyte liquid and perform the electrolyte rebalancing reaction without a packed catalyst bed.

[0031] During operation of the redox flow battery system 10, sensors and probes can monitor and control electrolyte chemical properties, such as electrolyte pH, concentration, SOC, etc. For example, as shown in FIG. 1 , sensors 62 and 60 can be positioned to monitor the state of the positive and negative electrolytes in the positive electrolyte chamber 52 and the negative electrolyte chamber 50, respectively. In another embodiment, sensors 62 and 60 can each include one or more electrolyte level sensors to indicate the level of electrolyte in the positive electrolyte chamber 52 and the negative electrolyte chamber 50, respectively. As another example, sensors 72 and 70, also shown in FIG. 1 , can monitor the state of the positive and negative electrolytes in the positive electrode compartment 22 and the negative electrode compartment 20, respectively. Sensors 72 and 70 can be pH probes, optical probes, pressure sensors, voltage sensors, etc. It will be understood that sensors can be positioned elsewhere throughout the redox flow battery system 10 to monitor electrolyte chemical and other properties.

[0032] For example, sensors may be positioned in an external acid tank (not shown) to monitor the acid level or pH of the external acid tank, and acid from the external acid tank may be supplied to the redox flow battery system 10 via an external pump (not shown) to reduce precipitate formation in the electrolyte. Additional external tanks and sensors may be installed to supply other additives to the redox flow battery system 10. For example, various sensors, including temperature, conductivity, and level sensors of the field hydration system, may send signals to the controller 88. Furthermore, the controller 88 may send signals to actuators, such as valves and pumps, of the field hydration system during hydration of the redox flow battery system 10. Sensor information may then be transmitted to the controller 88, which may, for example, operate pumps 30 and 32 to control electrolyte flow through the redox flow battery cells 18 or perform other control functions. In this manner, the controller 88 may respond to one or a combination of sensors and probes.

[0033] The redox flow battery system 10 may further include a source of H gas. In one embodiment, the source of H gas may include a separate, dedicated hydrogen gas storage tank. In the embodiment of FIG. 1 , H gas may be stored in and supplied from an integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supply additional H gas to the positive electrolyte chamber 52 and the negative electrolyte chamber 50. The integrated multi-chamber electrolyte storage tank 110 may alternately supply additional H gas to the inlets of the electrolyte rebalancing reactors 80 and 82. As one example, a mass flow meter or other flow control device (which may be controlled by the controller 88) may regulate the flow of H gas from the integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supplement the H gas generated within the redox flow battery system 10. For example, if a gas leak is detected in the redox flow battery system 10 or if the reduction reaction rate is too slow at low hydrogen partial pressure, H gas can be supplied from the integrated multi-chamber electrolyte storage tank 110 to rebalance the SOC of the electroactive materials in the positive and negative electrolytes. As one example, the controller 88 can supply H gas from the integrated multi-chamber electrolyte storage tank 110 in response to a measured change in pH or in response to a measured change in the SOC of the electrolyte or electroactive materials.

[0034] For example, an increase in pH in the negative electrolyte chamber 50 or anode compartment 20 may indicate that H2 is leaking from the redox flow battery system 10 and / or that the reaction rate is too slow at the available hydrogen partial pressure, and the controller 88 may increase the supply of H2 gas from the integrated multi-chamber electrolyte storage tank 110 to the redox flow battery system 10 in response to the increase in pH. As a further example, the controller 88 may supply H2 gas from the integrated multi-chamber electrolyte storage tank 110 in response to a pH change where the pH increases above a first threshold pH or decreases above a second threshold pH. In the case of an IFB, the controller 88 may supply additional H2 to increase the pH of the redox flow battery system 10. 3+This can increase the rate of ion reduction and proton production, thereby decreasing the pH of the positive electrolyte. Furthermore, the pH of the negative electrolyte can be increased by the amount of Fe crossing over from the positive electrolyte to the negative electrolyte. 3+ The pH of the negative electrolyte can be lowered by hydrogen reduction of the ions or by protons generated on the positive side crossing over into the negative electrolyte due to the proton concentration gradient and electrophoretic forces. In this way, the pH of the negative electrolyte is reduced by the release of Fe(OH)3 as Fe(OH). 3+ The risk of precipitation of ions (crossing over from the positive electrode compartment 22) can be reduced while remaining within a stable region.

[0035] Other control schemes can be implemented to control the supply rate of H gas from the integrated multi-chamber electrolyte storage tank 110 in response to changes in electrolyte pH or changes in electrolyte SOC detected by other sensors, such as an oxygen reduction potential (ORP) meter or optical sensor. Furthermore, the change in pH or SOC trigger action of the controller 88 can be based on a rate of change or a change measured over a period of time. The rate of change period can be predetermined or adjusted based on the time constant of the redox flow battery system 10. For example, if the recirculation rate is high, the period can be shortened, and the time constant can be small so that local changes in concentration (e.g., due to side reactions or gas leaks) can be measured quickly.

[0036] The controller 88 may further implement a control scheme based on the operating mode of the redox flow battery system 10. For example, the controller 88 may control the charging and discharging of the redox flow battery cells 18 to cause pre-forming of iron at the anode 26 during system conditioning (system conditioning may include an operating mode used to optimize the electrochemical performance of the redox flow battery system 10 outside of battery cycling). That is, during system conditioning, the controller 88 may adjust one or more operating conditions of the redox flow battery system 10 to plate iron metal on the anode 26 to improve battery charge capacity during subsequent battery cycling (thus, iron metal may be pre-formed for battery cycling). The controller 88 may also perform electrolyte rebalancing, as discussed above, to remove excess hydrogen gas from the redox flow battery system 10 and to pre-form Fe 3+ ion concentration. In this manner, preforming iron at the anode 26 and performing electrolyte rebalancing during system conditioning may increase the overall capacity of the redox flow battery cell 18 during battery cycling by mitigating iron plating loss. As used herein, battery cycling (also referred to as "charge cycling") may include alternating between charge and discharge modes of the redox flow battery system 10.

[0037] It will be understood that all components except the sensors 60 and 62 and the integrated multi-chamber electrolyte storage tank 110 (and components contained therein) can be considered to be included in the power module 120. As such, the redox flow battery system 10 can be described as including the power module 120 fluidly coupled to the integrated multi-chamber electrolyte storage tank 110 and communicatively coupled to the sensors 60 and 62. In some embodiments, the power module 120 and the multi-chamber electrolyte storage tank 110 can each be contained in a single housing (not shown), such that the redox flow battery system 10 can be contained as a single unit in a single location. It will further be understood that the positive electrolyte, the negative electrolyte, the sensors 60 and 62, the electrolyte rebalancing reactors 80 and 82, and the integrated multi-chamber electrolyte storage tank 110 (and components contained therein) can be considered to be included in the electrolyte subsystem 130. As such, the electrolyte subsystem 130 can supply one or more electrolytes to the redox flow battery cells 18 (and components contained therein).

[0038] The size (e.g., volume) of an integrated multi-chamber electrolyte storage tank (such as the integrated multi-chamber electrolyte storage tank 110) can determine the capacity of the redox flow battery system. If the redox flow battery is an IFB, the capacity of the battery can be further determined by the amount of iron ions that can be stored in the electrolyte storage tank. The capacity of the battery can be increased by increasing the concentration of iron salts in the electrolyte without increasing the size of the electrolyte storage tank. Increasing the capacity without changing the size of the electrolyte storage tank can increase the overall energy density of the system. High-energy density electrolytes can enable iron ion concentrations up to 3M by adding supporting salts such as ammonium and calcium salts, as described below. Traditionally, potassium chloride can be used as a supporting salt, and increasing the iron salt concentration can require a concomitant increase in potassium chloride concentration. However, potassium chloride may not remain completely soluble and / or stable at increased concentrations with increasing iron salt concentrations. Increasing the concentration of alternative supporting salts (e.g., ammonium chloride and calcium chloride) while minimizing potassium chloride may allow for increased solubility of the iron salt while keeping the supporting salt fully soluble in the electrolyte.

[0039] Referring now to FIG. 2 , plot 200 shows discharge capacity as a percentage of theoretical discharge capacity as a function of cycle number for an IFB cycled between two different SOCs (e.g., 20% to 80%). Arrow 208 indicates the direction of increasing discharge capacity along the y-axis of plot 200, and arrow 210 indicates the direction of increasing cycle number along the x-axis of plot 200. Trace 202 corresponds to an IFB constructed with a reference electrolyte (e.g., a baseline electrolyte). The reference electrolyte may have the same iron salt concentration as the high-energy-density electrolyte but may lack an additional supporting salt, such as an ammonium salt or a calcium salt. In this way, comparing an IFB constructed with the reference electrolyte to an IFB constructed with a high-energy-density electrolyte may highlight differences due to the addition of an ammonium salt or a calcium salt. Trace 204 corresponds to an IFB constructed with a first high-energy-density electrolyte. Trace 206 corresponds to an IFB constructed with a second high-energy-density electrolyte. Table 1 below shows the composition of the reference electrolyte with a range of compositions that may be used for the high energy density electrolytes, such as the first and second high energy density electrolytes. [Table 1] Concentrations of reference electrolyte and high energy density electrolyte

[0040] High energy density electrolytes may include iron salts, such as FeCl, in a concentration range from 1.7M to 3.0M. In one embodiment, the iron salt concentration of the high energy density electrolyte may be at least 2.0M. In addition to increasing the concentration of iron salt, high energy density electrolytes may include ammonium chloride (NHCl), and optionally calcium chloride (CaCl), to provide additional supporting salts. Potassium chloride (KCl) may be present up to 1.5M, or may be 1.5M or less. Additionally, while the table above lists chloride salts, other anions (e.g., SO4 2-) are considered within the scope of this application. The traces shown in Figure 2, as well as those included in the plots shown in Figures 4-10, may be collected from an IFB containing either a first or second high energy density electrolyte. The second high energy density electrolyte may differ from the first high energy density electrolyte by additionally including calcium chloride. Table 2 below lists the concentrations of the components of the first and second high energy density electrolytes. [Table 2] Compositions of the First and Second High Energy Density Electrolytes

[0041] Trace 202 in Figure 2 shows the decrease in discharge capacity of an IFB constructed with the reference electrolyte after the second cycle. The discharge capacity continues to decrease over subsequent cycles, dropping to 50% of the theoretical discharge capacity by the completion of five cycles. In contrast, traces 204 and 206 show that when the IFB is constructed with either the first or second high-energy-density electrolyte, the initial discharge capacity of the system is maintained at 100% of the theoretical discharge capacity for at least five cycles. Traces 202, 204, and 206 all correspond to systems using an electrolyte containing 2.5 M FeCl2. The addition of ammonium chloride, and optionally calcium chloride, to the high-energy-density electrolyte results in helping to maintain the discharge capacity of an IFB with a high iron salt concentration over multiple cycles. The discharge capacity of the IFB can be maintained because the electrolyte salts, including both iron and potassium salts, remain fully dissolved for at least five cycles.

[0042] Referring now to FIG. 3, plot 300 shows cell voltage (e.g., potential) as a function of time for an IFB configured with a reference electrolyte. Arrow 308 indicates increasing cell voltage along the y-axis of plot 300, and arrow 310 indicates increasing elapsed time along the x-axis of plot 300. Over the course of the experiment (e.g., increasing time), the IFB may be continuously cycled from 20% SOC to 80% SOC multiple times. For example, the period indicated by bracket 304 may correspond to the IFB system increasing from 20% SOC to 80% SOC, and the period indicated by bracket 306 may correspond to the IFB system decreasing from 80% SOC to 20% SOC. Trace 302 may correspond to the potential measured during charge / discharge of the IFB system, indicating unstable performance. The maximum charge cell voltage reached at 80% SOC may decrease over the course of multiple charge / discharge cycles, which may indicate electrolyte degradation.

[0043] As discussed above with respect to Figure 2, the discharge capacity of an IFB can decrease as the iron salt concentration increases beyond 2 M without the addition of a supporting salt such as ammonium chloride or calcium chloride as the IFB system undergoes successive charge and discharge cycles. This can be evidenced by trace 302, which shows instability in the cell potential measured over multiple charge / discharge events, as discussed above. This can be due to electrolyte degradation over time as a result of the insolubility of iron at 2.5 M in unsupported electrolyte.

[0044] Referring now to FIG. 4, plot 400 shows data plotted in trace 402 collected in a manner similar to that of FIG. 3, but using an IFB system incorporating a first high-energy density electrolyte as described above with respect to Table 2. Arrow 404 indicates increasing cell voltage along the y-axis of plot 400, and arrow 406 indicates increasing elapsed time along the x-axis of plot 400. The first high-energy density electrolyte may have a base composition similar to that of the reference electrolyte, but additionally includes ammonium chloride at a concentration of 1 M (whereas the reference electrolyte does not include ammonium chloride). The addition of ammonium chloride may increase the stability of the iron chloride in the electrolyte. Trace 402 corresponds to cell voltage measured as a function of time over the course of charge and discharge. Unlike the data shown in FIG. 3, trace 402 demonstrates more stable performance of the IFB over multiple consecutive charge / discharge cycles. Trace 402 shows that the cell voltage repeatedly cycles between more consistent maximum and minimum cell voltages during the six charge / discharge cycles shown compared to the reference electrolyte. The data shown in Figure 4 demonstrate the resistance of the first high-energy density electrolyte to degradation during battery operation and more stable battery performance, despite containing the same iron salt concentration as the IFB reference electrolyte measured in Figure 3.

[0045] Referring now to FIG. 5, plot 500 shows data plotted on traces collected in a manner similar to that of FIGS. 3 and 4, as described above with respect to Table 2, except using an IFB system incorporating a second high-energy density electrolyte. Arrow 504 indicates increasing cell voltage along the y-axis of plot 500, and arrow 506 indicates increasing elapsed time along the x-axis of plot 500. The second high-energy density electrolyte may be similar to the first high-energy density electrolyte, but may additionally contain calcium chloride at a concentration of 0.5 M, whereas the first high-energy density electrolyte does not contain calcium chloride. Trace 502 corresponds to the cell potential measured as a function of time over the course of charge and discharge. Similar to the data shown in FIG. 4, trace 502 shows a uniform and reproducible change between the two potential values as the IFB is charged and discharged over the period for which the data was collected, demonstrating more stable battery performance than that achieved with the reference electrolyte.

[0046] FIG. 6 shows a plot 600 illustrating cycle efficiency, including coulombic efficiency, voltaic efficiency, and energy efficiency, as a function of the number of charge / discharge cycles. Arrow 610 indicates the direction of increasing efficiency along the y-axis of plot 600, and arrow 612 indicates the direction of increasing cycle number along the x-axis of plot 600. The data shown in plot 600 may correspond to measurements collected using an IFB system incorporating a first high-energy-density electrolyte. Traces 602, 604, and 608 correspond to energy efficiency, voltaic efficiency, and coulombic efficiency, respectively. Apart from changes between the first two cycles and the last two cycles, the slopes of traces 602, 604, and 608 may be approximately zero. In other words, the efficiency of an IFB including a first high-energy-density electrolyte may remain relatively uniform over multiple cycles without degradation in charge capacity.

[0047] Referring now to FIG. 7 , plot 700 shows cycle efficiency, including coulombic efficiency, voltaic efficiency, and energy efficiency, as a function of the number of charge / discharge cycles. Arrow 708 indicates the direction of increasing efficiency along the y-axis of plot 700, and arrow 710 indicates the direction of increasing cycle number along the x-axis of plot 700. The data shown in plot 700 may be similar to the data shown in plot 600 of FIG. 6 , but collected using an IFB system incorporating a second, high-energy-density electrolyte. Traces 702, 704, and 706 correspond to energy efficiency, voltaic efficiency, and coulombic efficiency, respectively. Similar to traces 602, 604, and 608 of FIG. 6 , traces 702, 704, and 706 remain relatively flat, indicating electrolyte stability over multiple cycles. Furthermore, traces 702, 704, and 706 may demonstrate sustained stability, including through the final cycle shown in plot 700 for the second, high-energy-density electrolyte.

[0048] As explained above with respect to Figures 2-7, both the first and second high energy density electrolytes may be more stable than the reference electrolyte over multiple charge / discharge cycles. While both the first high energy density electrolyte and the second high energy density electrolyte are relatively stable, particularly compared to the reference electrolyte, there may also be differences in stability between the first and second embodiments of the high energy density electrolyte, which can be seen in Figures 8-9.

[0049] Referring now to FIG. 8, plot 800 shows the percentage of theoretical discharge capacity as a function of cycle number. Arrow 806 indicates the direction of increasing percentage of theoretical discharge capacity along the y-axis of plot 800, and arrow 808 indicates the direction of increasing cycle number along the x-axis of plot 800. Trace 804 corresponds to a first high energy density electrolyte and may be the same as trace 204 of FIG. 2. Trace 802 corresponds to a second high energy density electrolyte and may be the same as trace 206 of FIG. 2. However, traces 802 and 804 are plotted over 16 cycles in plot 800, as opposed to the 5 cycles plotted in plot 200. Trace 804 maintains a steady discharge capacity for 4 cycles, after which the discharge capacity fluctuates. Trace 802 maintains a steady discharge capacity for at least 10 cycles, after which the discharge capacity fluctuates. As is evident from a comparison between traces 802 and 804, calcium chloride included in the second embodiment of the high energy density electrolyte is able to maintain the discharge capacity of the IFB for more cycles.

[0050] Referring now to FIG. 9 , plot 900 shows potential as a function of time. Arrow 906 indicates increasing cell voltage along the y-axis of plot 900, and arrow 908 indicates increasing elapsed time along the x-axis of plot 900. Trace 902 corresponds to a first high energy density electrolyte and may be the same as trace 402 of FIG. 4 . Trace 904 corresponds to a second high energy density electrolyte and may be the same as trace 502 of FIG. 5 . Traces 902 and 904 may be substantially the same, indicating that the addition of calcium chloride to a high energy density electrolyte may have little effect on the potential of an IFB at low charge densities.

[0051] In this way, a high-energy-density electrolyte with an iron concentration of up to 3M, in addition to supporting salts including calcium and ammonium salts, can increase the capacity of an IFB system without requiring an increase in electrolyte volume. As a result, the footprint of the IFB system can remain compact. Adding auxiliary supporting salts, such as calcium and ammonium salts, allows for an increase in the total iron concentration of the IFB electrolyte without adversely affecting battery performance. Ammonium and / or calcium salts can maintain the desirable low-cost and low-toxicity characteristics of the IFB electrolyte. Furthermore, ammonium and calcium salts can be added to both the negative electrolyte in the negative compartment of the IFB and the positive electrolyte in the positive compartment of the IFB, with the negative electrolyte adjusted to have a lower pH than the positive electrolyte. Maintaining a covalent composition of the two electrolytes minimizes the effects of ion crossover.

[0052] The present disclosure also provides an electrolyte for a redox flow battery, comprising a redox-active species dissolved in the electrolyte and having a concentration greater than 2.0 M, and a plurality of dissolved supporting salts, including potassium salts, ammonium salts, calcium salts, and manganese salts. In a first embodiment of the system, the redox-active species is an iron salt. In a second embodiment of the system, optionally including the first embodiment, the redox-active species has a concentration range of 2.0 M to 3.0 M. In a third embodiment of the system, optionally including one or both of the first and second embodiments, the potassium salt is potassium chloride having a concentration range of 0.5 M to 1.5 M. In a fourth embodiment of the system, optionally including one or more or each of the first through third embodiments, the ammonium salt is ammonium chloride having a concentration range of 0.5 M to 2.5 M. In a fifth embodiment of the system, optionally including one or more or each of the first through fourth embodiments, the calcium salt is calcium chloride having a concentration range of 0 M to 1.5 M. In a sixth embodiment of a system optionally comprising one or more or each of the first through fifth embodiments, the manganese salt is manganese chloride having a concentration range of 0 M to 0.25 M. In a seventh embodiment of a system optionally comprising one or more or each of the first through sixth embodiments, the system further comprises boric acid having a concentration range of 0 M to 0.25 M. In an eighth embodiment of a system optionally comprising one or more or each of the first through seventh embodiments, the electrolyte is an aqueous electrolyte.

[0053] The present disclosure also provides a support for a redox flow battery, including an electrolyte solution, the composition of which is shared between the electrolyte in the anode compartment and the electrolyte in the cathode compartment, the electrolyte solution including dissolved iron at a concentration of at least 2.0 M and a dissolved supporting salt including an ammonium salt. In a first embodiment of the system, the electrolyte solution in the anode compartment has a lower pH than the electrolyte solution in the cathode compartment, and the dissolved iron and dissolved supporting salt remain fully dissolved in both the anode compartment and the cathode compartment. In a second embodiment of the system, optionally including the first embodiment, the dissolved supporting salt further includes a calcium salt, and when the dissolved supporting salt includes the calcium salt, the discharge capacity of the redox flow battery is more uniform over multiple cycles than when the solution does not include the calcium salt. In a third embodiment of a system optionally including one or both of the first and second embodiments, the dissolved supporting salt further comprises a calcium salt, and when the dissolved supporting salt comprises a calcium salt and an ammonium salt, the discharge capacity of the redox flow battery is more uniform over multiple cycles than when the dissolved supporting salt does not comprise a calcium salt or an ammonium salt. In a fourth embodiment of a system optionally including one or more or each of the first through third embodiments, the dissolved supporting salt further comprises a calcium salt, and the discharge capacity of the redox flow battery remains stable over at least 10 cycles. In a fifth embodiment of a system optionally including one or more or each of the first through fourth embodiments, the dissolved supporting salt further comprises potassium chloride and manganese chloride.

[0054] The present disclosure also provides a support for an electrolyte composition for a redox flow battery, the support comprising an iron salt having a concentration of at least 2.0 M, an ammonium salt having a concentration of 0.5 M to 2.5 M, and an additional supporting salt at a concentration of 2.5 M or less. In a first embodiment of the system, the iron salt participates in a redox reaction during operation of the redox flow battery, and the ammonium salt and the additional supporting salt do not participate in a redox reaction during operation of the redox flow battery. In a second embodiment of the system, optionally including the first embodiment, the maximum charge cell voltage of the redox flow battery decreases with successive cycles when the ammonium salt is not present in the electrolyte composition. In a third embodiment of the system, optionally including one or both of the first and second embodiments, the additional supporting salt comprises a calcium salt at a concentration of up to 1.5 M. In a fourth embodiment of the system, optionally including one or more or each of the first through third embodiments, the redox flow battery is an iron redox flow battery.

[0055] The following claims particularly point out certain combinations and subcombinations that are deemed novel and unobvious. These claims may refer to "an" element or "first" element or equivalents. Such claims should be understood to include the incorporation of one or more such elements, and do not require or exclude two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and / or properties may be claimed through amendment of the claims or through the presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope from the original claims, are also deemed to be encompassed within the subject matter of this disclosure.

Claims

1. An electrolyte for a redox flow battery, a redox active species dissolved in the electrolyte and having a concentration greater than 2.0 M; a plurality of dissolved supporting salts including potassium salts, ammonium salts, calcium salts, and manganese salts.

2. 10. The electrolyte of claim 1, wherein the redox active species is an iron salt.

3. 10. The electrolyte of claim 1, wherein the redox active species is in a concentration range of 2.0M to 3.0M.

4. 2. The electrolyte of claim 1, wherein the potassium salt is potassium chloride in a concentration range of 0.5M to 1.5M.

5. 2. The electrolyte of claim 1, wherein the ammonium salt is ammonium chloride in a concentration range of 0.5M to 2.5M.

6. 2. The electrolyte of claim 1, wherein the calcium salt is calcium chloride in a concentration range of 0M to 1.5M.

7. 2. The electrolyte of claim 1, wherein the manganese salt is manganese chloride in a concentration range of 0M to 0.25M.

8. 10. The electrolyte of claim 1 further comprising boric acid in a concentration range of 0M to 0.25M.

9. The electrolyte of claim 1 , wherein the electrolyte is an aqueous electrolyte.

10. A redox flow battery, 1. A redox flow battery comprising: an electrolyte solution, the composition of which is shared between an electrolyte in an anode compartment and an electrolyte in a cathode compartment, the electrolyte solution comprising dissolved iron at a concentration of at least 2.0 M and a dissolved supporting salt comprising an ammonium salt.

11. 11. The redox flow battery of claim 10, wherein the electrolyte solution in the anode compartment has a lower pH than the electrolyte solution in the cathode compartment, and the dissolved iron and the dissolved supporting salt remain fully dissolved in both the anode compartment and the cathode compartment.

12. 11. The redox flow battery of claim 10, wherein the dissolved supporting salt further comprises a calcium salt, and when the dissolved supporting salt comprises the calcium salt, the discharge capacity of the redox flow battery is more uniform over multiple cycles than when the solution does not comprise the calcium salt.

13. 11. The redox flow battery of claim 10, wherein the dissolved supporting salt further comprises a calcium salt, and when the dissolved supporting salt comprises the calcium salt and the ammonium salt, the discharge capacity of the redox flow battery is more uniform over multiple cycles than when the dissolved supporting salt does not comprise the calcium salt or the ammonium salt.

14. 11. The redox flow battery of claim 10, wherein the dissolved supporting salt further comprises a calcium salt, and the discharge capacity of the redox flow battery remains stable for at least 10 cycles.

15. 11. The redox flow battery of claim 10, wherein the dissolved supporting salt further comprises potassium chloride and manganese chloride.

16. An electrolyte composition for a redox flow battery, comprising: an iron salt at a concentration of at least 2.0 M; an ammonium salt at a concentration of 0.5M to 2.5M; and an additional supporting salt at a concentration of 2.5M or less.

17. 17. The electrolyte composition of claim 16, wherein the iron salt participates in a redox reaction during operation of the redox flow battery, and the ammonium salt and the additional supporting salt do not participate in a redox reaction during operation of the redox flow battery.

18. 17. The electrolyte composition of claim 16, wherein the maximum charge cell voltage of the redox flow battery decreases with successive cycles when the ammonium salt is not present in the electrolyte composition.

19. 17. The electrolyte composition of claim 16, wherein the additional supporting salt comprises a calcium salt at a concentration of up to 1.5M.

20. 17. The electrolyte composition of claim 16, wherein the redox flow battery is an iron redox flow battery.

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