High energy density electrolyte

Abstract

Systems and methods are provided for an electrolyte for a flow battery that includes a redox active material and a plurality of support salts dissolved in the electrolyte. The concentration of the redox active material is greater than 2.0 M, and the plurality of dissolved support salts include potassium and ammonium salts, calcium salts, and manganese salts.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application No. 63 / 369,728, entitled “HIGH ENERGYDENSITY ELECTROLYTE,” filed on July 28, 2022. The entire contents of the above-identified application are hereby incorporated by reference for all purposes. Technical Field

[0003] The present description relates generally to electrolytes for redox flow batteries. Background Art

[0004] Redox flow batteries are suitable for grid scale storage applications due to their ability to scale power and capacity independently and to be able to charge and discharge for thousands of cycles with lower performance losses than conventional battery technologies. Iron hybrid redox flow batteries are particularly attractive due to the incorporation of low-cost materials in the battery stack. Iron redox flow batteries (IFBs) rely on iron, salts, and water as electrolytes. One way to increase the capacity of an IFB without increasing the size of the IFB (e.g., the size of the electrolyte tank) is to increase the concentration of iron salts dissolved in the electrolyte.

[0005] Attempts to increase the concentration of iron salts in the electrolyte are limited by the inherent solubility of the different salts contained in the IFB electrolyte. As part of a conventional IFB electrolyte solution, iron salts at a concentration of 1.7 M can be used in the electrolyte. Alternatively, iron salt concentrations up to 5.5 M have been reported in the field of ferroelectric deposition. However, electrolytes used for ferroelectric deposition or any other electrolyte without sufficient electrochemically inert supporting salts may not be easily transferred to the IFB system, otherwise it will also lead to an undesirable reduction in coulombic efficiency. In addition, the electrodeposition electrolyte may be formulated for use at high temperatures (e.g., above 60°C) and is therefore incompatible with auxiliary components of the pipeline used for the IFB. Summary of the invention

[0006] The inventors have recognized the above-mentioned shortcomings in previous strategies for increasing the iron salt concentration in the electrolyte of IFB, and have developed an electrolyte composition that at least partially overcomes these shortcomings. In one example, the electrolyte for a redox flow battery may include a redox active substance dissolved in the electrolyte and having a concentration of at least 2.0M in addition to a plurality of dissolved supporting salts including potassium salts, ammonium salts, calcium salts, and manganese salts. The electrolyte composition can allow the capacity of the IFB to be increased without increasing the size of the electrolyte tank or sacrificing battery stability. The electrolyte composition can be used with existing IFB components (e.g., battery stacks, pipes, pumps, etc.), thereby allowing the electrolyte composition described herein to be used economically and effectively to replace conventional electrolytes. In addition, the electrolyte can maintain an aqueous electrolyte, and the composition can maintain the valuable low-cost, low-toxicity properties in the IFB electrolyte.

[0007] It should be understood that the above summary is provided to introduce in simplified form a series of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is uniquely defined by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 A schematic diagram of an example redox flow battery system including an electrolyte is shown.

[0009] Figure 2 A graph showing the percentage of theoretical discharge capacity as a function of the number of charge cycles of the IFB is shown, comparing the results from the reference electrolyte, the first high energy density electrolyte, and the second high energy density electrolyte.

[0010] Figure 3 A graph of the potential of a reference electrolyte as a function of time is shown.

[0011] Figure 4 A graph of the potential of a first high energy density electrolyte as a function of time is shown.

[0012] Figure 5 A graph of the potential of the second highest energy density electrolyte as a function of time is shown.

[0013] Figure 6 A graph showing the efficiency of a first high energy density electrolyte as a function of the number of charging cycles.

[0014] Figure 7 A graph showing the efficiency of the second highest energy density electrolyte as a function of the number of charging cycles.

[0015] Figure 8 A graph showing the percentage of theoretical discharge capacity as a function of the number of charge cycles in an IFB is shown, comparing the results for a first high energy density electrolyte to a second high energy density electrolyte.

[0016] Fig. 9 A graph comparing voltage curves over time for a first high energy density electrolyte and a second high energy density electrolyte is shown. DETAILED DESCRIPTION

[0017] The following description relates to the composition of a high energy density electrolyte for use in an IFB. The electrolyte may be used in an IFB, such as Figure 1 Schematically depicted in . The energy density of the IFB electrolyte can be increased by increasing the concentration of dissolved iron species in the electrolyte. By adjusting the supporting salt in the electrolyte composition, the increase in the iron salt concentration of the high energy density electrolyte described herein can be achieved without sacrificing the performance of the IFB. The supporting salt herein may refer to a salt that does not participate in the redox reaction contained in the electrolyte of the redox flow battery. For example, in a first example of a high energy density electrolyte, ammonium chloride can be added to the baseline electrolyte. In a second example of a high energy density electrolyte, ammonium chloride and calcium chloride can be added. In addition to the iron salt, the baseline electrolyte may also include potassium chloride, manganese chloride and boric acid. Figure 2 A graph comparing the percentage of theoretical discharge capacity over multiple cycles for IFBs of the first and second embodiments comprising a baseline electrolyte (as a reference electrolyte) and a high energy density electrolyte is shown. Figures 3 to 5 The electrolytes listed above are further compared by graphs showing the potential of each electrolyte as a function of time. Figures 6 to 7 The first and second examples of high energy density electrolytes are further characterized by showing graphs comparing the efficiency of each electrolyte as a function of the number of charging cycles. Figure 8 The performance of the first embodiment of the high energy density electrolyte and the second embodiment of the high energy density electrolyte are emphasized. For example, Figure 8 It further shows Figure 2 A graph of the data is shown in FIG, but no data from the baseline electrolyte is available. Fig. 9 Plot an overlay of the voltaic curves over time.

[0018] like Figure 1 As shown, in the redox flow battery system 10, the negative electrode 26 may be referred to as a plating electrode, and the positive electrode 28 may be referred to as a redox electrode. The negative electrolyte within the plating side (e.g., negative electrode compartment 20) of the redox flow battery cell 18 may be referred to as a plating electrolyte, and the positive electrolyte on the redox side (e.g., positive electrode compartment 22) of the redox flow battery cell 18 may be referred to as a redox electrolyte.

[0019] "Anode" refers to an electrode where an electroactive material loses electrons, and "cathode" refers to an electrode where an electroactive material gains electrons. During battery charging, the negative electrolyte gains electrons at the negative electrode 26, and the negative electrode 26 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 the negative electrode 26 may be referred to as the anode electrolyte and anode of the electrochemical reaction, respectively, while the positive electrolyte and the positive electrode 28 may be referred to as the cathode electrolyte and cathode of the electrochemical reaction, respectively. During battery charging, the negative electrolyte and the negative electrode 26 may be referred to as the cathode electrolyte and cathode of the electrochemical reaction, respectively, while the positive electrolyte and the positive electrode 28 may be referred to as the anode electrolyte and anode of the electrochemical reaction, respectively. For simplicity, the terms "positive" and "negative" are used herein to refer to electrodes, electrolytes, and electrode compartments in a redox flow battery system.

[0020] An example of a hybrid redox flow battery is an all-iron redox flow battery (IFB), in which the electrolyte contains iron ions in the form of an iron salt (e.g., FeCl2, FeCl3, etc.), and in which the negative electrode 26 contains metallic iron. For example, at the negative electrode 26, divalent iron (Fe 2+ ) gains two electrons during battery charging and converts to iron metal (Fe 0 ) is electroplated onto the negative electrode 26, and Fe 0 During battery discharge, it loses two electrons and redissolves into Fe 2+ At the positive electrode 28, Fe 2+ During battery charging, it loses electrons to form ferric iron (Fe 3+ ), and Fe 3+ During battery discharge, electrons are gained to form Fe 2+ The electrochemical reactions are summarized in equations (1) and (2), where the forward reaction (from left to right) indicates the electrochemical reaction during battery charging, and the reverse reaction (from right to left) indicates the electrochemical reaction during battery discharging:

[0021]

[0022]

[0023] As discussed above, the negative electrolyte used in IFBs can provide sufficient amounts of Fe 2+ , so that during battery charging, Fe 2+ can accept two electrons from the negative electrode 26 to form Fe 0 During the discharge of the battery, the electroplated Fe 0Can lose two electrons, thus ionizing into Fe 2+ and dissolves back into the electrolyte. The equilibrium potential of the above reaction is -0.44V, so this reaction provides the negative terminal for the desired system. On the positive side of the IFB, the electrolyte provides Fe during battery charging. 2+ , which loses electrons and oxidizes to Fe 3+ During battery discharge, the electrolyte provides Fe 3+ By absorbing electrons supplied from the positive electrode 28, it becomes Fe 2+ The equilibrium potential for this reaction is +0.77 V, creating a positive terminal for the desired system.

[0024] Compared to other battery types that utilize non-regenerative electrolytes, IFBs can provide the ability to charge and recharge the electrolyte therein. Charging can be achieved by applying a current across electrodes 26 and 28 via terminals 40 and 42, respectively. Negative electrode 26 can be electrically coupled to the negative side of a voltage source via terminal 40 so that electrons can be transferred to the negative electrolyte via positive electrode 28 (e.g., when Fe 2+ Oxidized to Fe in the positive electrolyte in the positive electrode compartment 22 3+ The electrons provided to the negative electrode 26 can reduce the Fe 2+ To form Fe at the (electroplating) substrate 0 , so that Fe 2+ Electroplating onto the negative electrode 26 .

[0025] When Fe 0 Remain available for oxidation of the negative electrode electrolyte, while Fe 3+ Discharge can continue while the positive electrolyte remains available for reduction. For example, additional Fe can be provided via an external source (e.g., external positive electrolyte chamber 52) by increasing the concentration or volume of the positive electrolyte in the positive electrode compartment 22 side of the redox flow battery cell 18. 3+ ions to maintain Fe 3+ More commonly, during discharge, Fe 0 The availability of Fe may be problematic in IFB systems where the Fe 0 The charge capacity may depend on the Fe content in the negative electrode compartment 20. 2+ For example, additional Fe may be provided via an external source (e.g., external negative electrode electrolyte chamber 50). 2+ ions to increase the concentration or volume of the negative electrolyte in the negative electrode compartment 20 side of the redox flow battery cell 18 to maintain the Fe 2+It can be understood that increasing the concentration of iron in the positive and negative electrolytes can increase the capacity of the IFB system without increasing the volume of the electrolyte. In this way, the energy density of the IFB system can be increased.

[0026] In an IFB, depending on the state of charge (SOC) of the IFB system, the positive electrolyte may include ferrous iron, ferric iron, ferric iron complexes, or any combination thereof, while the negative electrolyte may include ferrous iron or ferrous iron complexes. As previously mentioned, the use of iron ions in the negative and positive electrolytes allows the same electrolyte material to be used on both sides of the redox flow battery cell 18, which can reduce electrolyte cross-contamination and can improve the efficiency of the IFB system, thereby producing less electrolyte replacement compared to other redox flow battery systems. Adding a supporting salt to the electrolyte as described below can allow the iron concentration in the electrolyte solution to be increased. The supporting salt can be a salt that increases the conductivity of the electrolyte solution and further contributes to the stability of the redox active salt (e.g., FeCl2) but is not oxidized or reduced during the operation of the redox flow battery.

[0027] Efficiency loss in IFBs may be caused by electrolyte crossover through the separator 24 (e.g., ion exchange membrane barrier, microporous membrane, etc.). For example, Fe 3+ Ions can pass through Fe 3+ The Fe 3+ Ions penetrating separator 24 and exchanging to negative electrode compartment 20 may result in Coulombic efficiency losses. 3+ The exchange of ions from the low pH redox side (e.g., the more acidic positive electrode compartment 22) to the high pH plating side (e.g., the less acidic negative electrode compartment 20) can result in the precipitation of Fe(OH)3. Precipitation of Fe(OH)3 can degrade the separator 24 and result in permanent battery performance and efficiency losses. For example, Fe(OH)3 precipitates can chemically contaminate the organic functional groups of the ion exchange membrane or physically block the micropores of the ion exchange membrane. In either case, the membrane ohmic resistance may increase over time and battery performance may decrease due to the Fe(OH)3 precipitates. The precipitates can be removed by washing the IFB with acid, but the ongoing maintenance and downtime may be detrimental to commercial battery applications. In addition, cleaning may depend on regular preparation of the electrolyte, contributing to additional processing costs and complexity. Alternatively, the addition of specific organic acids to the positive and negative electrolytes in response to changes in electrolyte pH can mitigate precipitate formation during battery charge and discharge cycles without increasing overall cost. In addition, implementation of Fe(OH)3 inhibition 3+ The ion exchange membrane barrier also mitigates fouling.

[0028] Additional Coulombic efficiency losses may be caused by H + The reduction of (eg, protons) and subsequent formation of H 2 gas is caused by the protons in the negative electrode compartment 20 reacting with the electrons supplied at the electroplated iron metal of the negative electrode 26 to form H 2 gas.

[0029] IFB electrolytes (e.g., FeCl2, FeCl3, FeSO4, Fe2(SO4)3, etc.) are readily available and can be produced at low cost. In one example, the IFB electrolyte may be formed of ferrous chloride (FeCl2), potassium chloride (KCl), manganese (II) chloride (MnCl2), and boric acid (H3BO3). In addition, ammonium chloride (NH4Cl) and calcium chloride (CaCl2) may be included in the electrolyte, and Fe in the solution may be allowed to react with the electrolyte. 2+ / 3+ The concentration of is increased above the concentration that is generally considered to be stable. In this way, a high energy density electrolyte can be formed. IFB electrolytes can provide higher recycling value because the same electrolyte can be used for the negative electrode electrolyte and the positive electrode electrolyte, thus reducing cross-contamination problems compared to other systems. In addition, due to the electronic configuration of iron, iron can be solidified into a substantially uniform solid structure during electroplating of iron onto the negative electrode substrate. For zinc and other metals commonly used in hybrid redox batteries, solid dendrite-like structures may be formed during electroplating. Compared with other redox flow batteries, the stable electrode morphology of the IFB system can improve the efficiency of the battery. In addition, compared with other redox flow battery electrolytes, the iron redox flow battery can reduce the use of toxic raw materials and can operate at a relatively neutral pH. Therefore, compared with all other current advanced redox flow battery systems in production, the IFB system can reduce environmental hazards.

[0030] Continue to refer Figure 1 , a schematic illustration 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 a negative electrode compartment 20, a separator 24, and a positive electrode compartment 22. The separator 24 may include an electrically insulating ion-conducting barrier that prevents substantial mixing of the positive and negative electrolytes while allowing specific ions to conduct therethrough. For example, and as discussed above, the separator 24 may include an ion exchange membrane and / or a microporous membrane.

[0031] Negative electrode compartment 20 may include a negative electrode 26, and the negative electrolyte may include an electroactive material. Positive electrode compartment 22 may include a positive electrode 28, and the positive electrolyte may include an electroactive material. In some examples, multiple redox flow battery cells 18 may be combined in series or in parallel to produce a higher voltage or current in the redox flow battery system 10.

[0032] Figure 1 Further illustrated are negative electrolyte pump 30 and positive electrolyte pump 32, both of which are used to pump electrolyte solution through the redox flow battery system 10. The electrolyte is stored in one or more tanks external to the battery cell and is pumped through the negative electrode compartment 20 side and the positive electrode compartment 22 side of the redox flow battery cell 18 via the negative electrolyte pump 30 and the positive electrolyte pump 32, respectively.

[0033] The redox flow battery system 10 may also include a first bipolar plate 36 and a second bipolar plate 38, each of which is positioned along a side of the negative electrode 26 and the positive electrode 28 facing the rear (e.g., opposite to 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 examples, the bipolar plates 36 and 38 may be arranged close to the electrodes 26 and 28 but spaced apart from the electrodes, and housed in the respective electrode compartments 20 and 22. In either case, the bipolar plates 36 and 38 may be electrically coupled to the terminals 40 and 42, respectively, via direct contact with the terminals 40 and 42 or through the negative electrode 26 and the positive electrode 28, respectively. The IFB electrolyte may be transported to the reaction sites at the negative electrode 26 and the positive electrode 28 by the first bipolar plate 36 and the second bipolar plate 38, which is caused by the conductive properties of the materials of the bipolar plates 36 and 38. Electrolyte flow may also be assisted by negative and positive electrolyte pumps 30, 32, promoting forced convection through the redox flow battery cell 18. Reacted electrochemical species may also be directed away from reaction sites by a combination of forced convection and the presence of the first and second bipolar plates 36, 38.

[0034] like Figure 1 As illustrated, the redox flow battery cell 18 may also include a negative battery negative terminal 40 and a battery positive 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, a reverse redox reaction 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. The potential difference across the battery may be maintained by the electrochemical redox reaction in the positive electrode compartment 22 and the negative electrode compartment 20, and a current may be induced through the current collector while the reaction continues. The amount of energy stored by the redox battery may be limited by the amount of electroactive material available for discharge in the electrolyte, which depends on the total volume of the electrolyte and the solubility of the electroactive material.

[0035] The redox flow battery system 10 may further include an integrated multi-chamber electrolyte tank 110. The multi-chamber electrolyte tank 110 may be separated by a separator 98. The separator 98 may form multiple chambers within the multi-chamber electrolyte tank 110 so that both the positive electrolyte and the negative electrolyte may be contained within a single tank. The negative electrolyte chamber 50 contains a negative electrolyte containing an electroactive material, and the positive electrolyte chamber 52 contains a positive electrolyte containing an electroactive material. The separator 98 may be positioned within the multi-chamber electrolyte tank 110 to produce a desired volume ratio between the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In one example, the separator 98 may be positioned to set the volume ratio of the negative electrolyte chamber 50 to the positive electrolyte chamber 52 according to the stoichiometric ratio between the negative redox reaction and the positive redox reaction. Figure 1 Further illustrated is a fill level 112 of the multi-chamber electrolyte tank 110 , which may indicate the liquid level in each tank compartment. Figure 1 Also shown is a gas headspace 90 located above the fill level 112 of the negative electrolyte chamber 50, and a gas headspace 92 located above the fill level 112 of the positive electrolyte chamber 52. The gas headspace 92 can be used to store H2 gas generated by the operation of the redox flow battery (e.g., due to proton reduction and iron corrosion side reactions) and delivered to the multi-chamber electrolyte storage tank 110 along with the return electrolyte from the redox flow battery cell 18. The H2 gas can spontaneously separate at the gas-liquid interface (e.g., fill level 112) within the multi-chamber electrolyte storage tank 110, thereby preventing the need for an additional gas-liquid separator as part of the redox flow battery system 10. Once separated from the electrolyte, the H2 gas can fill the gas headspaces 90 and 92. Therefore, the stored H2 gas can help to purge other gases from the multi-chamber electrolyte storage tank 110, thereby acting as an inert gas layer for reducing oxidation of electrolyte species, which can help to reduce redox flow battery capacity loss. In this way, the use of the integrated multi-chamber electrolyte tank 110 can eliminate the separate negative electrolyte tank and positive electrolyte tank, hydrogen storage tank and gas-liquid separator commonly used in conventional redox flow battery systems, thereby simplifying the system design, reducing the physical footprint of the redox flow battery system 10, and reducing the system cost.

[0036] Figure 1Also shown is an overflow hole 96 which may form an opening in the separator 98 between the gas head spaces 90 and 92 and may provide a means of equalizing the gas pressure between the chambers 50 and 52. The overflow hole 96 may be positioned at a threshold height above the fill height 112. The overflow hole 96 may also 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 battery swap. In the case of an all-iron redox flow battery system, the same electrolyte (Fe 2 O 2 ) is used in the negative and positive electrode compartments 20 and 22. 2+ ), so the overflow of electrolyte between the negative electrolyte chamber 50 and the positive electrolyte chamber 52 can reduce the overall system efficiency, but the overall electrolyte composition, battery module performance and battery module capacity can be maintained. Flange fittings can be used for all pipe connections to and from the inlet and outlet of the multi-chamber electrolyte storage tank 110 to maintain a continuous pressurized state without leakage. The multi-chamber electrolyte storage tank 110 may include at least one outlet from each of the negative electrolyte chamber 50 and the positive electrolyte chamber 52, and at least one inlet to each of the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In addition, one or more outlet connections may be provided from the gas head spaces 90 and 92 for directing H2 gas to the rebalance reactor or cells 80 and 82.

[0037] although Figure 110. Although not shown in the figure, the integrated multi-chamber electrolyte tank 110 may further include one or more heaters thermally coupled to each of the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In an alternative example, only one of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 may include one or more heaters. In the case where only the positive electrolyte chamber 52 includes one or more heaters, the negative electrolyte can be heated by transferring the heat generated at the redox flow battery cell 18 to the negative electrolyte. In this way, the redox flow battery cell 18 can heat and promote the temperature regulation of the negative electrolyte. One or more heaters can be actuated by the controller 88 to independently or jointly regulate the temperature of the negative electrolyte chamber 50 and the positive electrolyte chamber 52. For example, in response to the electrolyte temperature decreasing below a threshold temperature, the controller 88 can increase the power supplied to the one or more heaters so that the heat flux to the electrolyte can be increased. The electrolyte temperature can be indicated by one or more temperature sensors (e.g., sensors 60 and 62) installed at the multi-chamber electrolyte tank 110. For example, one or more heaters may include coil-type heaters or other immersion heaters immersed in the electrolyte, or surface-covered heaters that transfer heat through the walls of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 in a conductive manner to heat the fluid therein. Other known types of tank heaters may be employed without departing from the scope of the present disclosure. In addition, the controller 88 may deactivate one or more heaters in the negative electrolyte chamber 50 and the positive electrolyte chamber 52 in response to the liquid level decreasing below the solid fill threshold level. In other words, in some examples, the controller 88 may activate one or more heaters in the negative electrolyte chamber 50 and the positive electrolyte chamber 52 only in response to the liquid level increasing above the solid fill threshold level. In this way, activation of one or more heaters in the absence of sufficient liquid in the negative electrolyte chamber 50 and / or the positive electrolyte chamber 52 may be avoided, thereby reducing the risk of overheating or burning out of one or more heaters.

[0038] Still further, one or more inlet connections may be provided to each of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 from an on-site hydration system (not shown). In this way, the on-site hydration system may facilitate the commissioning of the redox flow battery system 10, including installing, filling, and hydrating the redox flow battery system 10 at the final use location. In addition, before the redox flow battery system 10 is commissioned at the final use location, the redox flow battery system 10 may be dry assembled at a battery manufacturing facility different from the final use location without filling and hydrating the redox flow battery system 10, and then the redox flow battery system 10 is delivered to the final use location. In one example, the final use location may correspond to the location where the redox flow battery system 10 will be installed and used for on-site energy storage. In other words, the redox flow battery system 10 may be designed so that once installed and hydrated at the final use location, the location of the redox flow battery system 10 may become fixed, and the redox flow battery system 10 may no longer be considered a portable, dry system. Thus, from the end user's perspective, the dry, portable redox flow battery system 10 can be delivered on site, after which the redox flow battery system 10 can be installed, hydrated, and commissioned. Prior to hydration, the redox flow battery system 10 can be referred to as a dry, portable system, and the redox flow battery system 10 contains no or no water and a wet electrolyte. Once hydrated, the redox flow battery system 10 can be referred to as a wet, non-portable system, and the redox flow battery system 10 contains a wet electrolyte.

[0039] like Figure 1 To further illustrate, the electrolyte solution primarily stored in the multi-chamber electrolyte storage tank 110 may be pumped through the entire redox flow battery system 10 via the negative electrolyte pump 30 and the positive electrolyte pump 32. The electrolyte stored in the negative electrolyte chamber 50 may be pumped through the negative electrode 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 may be pumped through the positive electrode compartment 22 side of the redox flow battery cell 18 via the positive electrolyte pump 32.

[0040] The electrolyte rebalance reactors 80 and 82 may be connected in series or in parallel with the recirculation flow paths of the electrolyte at the negative and positive sides of the redox flow battery cells 18 in the redox flow battery system 10, respectively. One or more rebalance reactors may be connected in series with the recirculation flow paths of the electrolyte at the negative and positive sides of the battery, and other rebalance reactors may be connected in parallel to achieve redundancy (e.g., to provide services to the rebalance reactors without disrupting the battery and rebalancing operations) and to enhance rebalancing capabilities. In one example, the electrolyte rebalance reactors 80 and 82 may be placed in the return paths leading from the negative electrode compartment 20 and the positive electrode compartment 22 to the negative electrolyte chamber 50 and the positive electrolyte chamber 52, respectively.

[0041] Electrolyte rebalance reactors 80 and 82 may be used to rebalance electrolyte charge imbalances that occur in the redox flow battery system 10 due to side reactions, ion exchange, etc., as described herein. In one example, the electrolyte rebalance reactors 80 and 82 may include trickle bed reactors, in which H2 gas and electrolyte may contact at the catalyst surface in the packed bed to perform an electrolyte rebalance reaction. In other examples, the rebalance reactors 80 and 82 may include overflow reactors that are capable of contacting H2 gas and electrolyte liquid and performing an electrolyte rebalance reaction without a packed catalyst bed.

[0042] During operation of the redox flow battery system 10, sensors and detectors may monitor and control the chemical properties of the electrolyte, such as electrolyte pH, concentration, SOC, etc. For example, Figure 1 As illustrated, sensors 62 and 60 may be positioned to monitor the positive electrolyte and negative electrolyte conditions at positive electrolyte chamber 52 and negative electrolyte chamber 50, respectively. In another example, sensors 62 and 60 may each include one or more electrolyte level sensors to indicate the level of electrolyte in positive electrolyte chamber 52 and negative electrolyte chamber 50, respectively. As another example, also as Figure 1 As illustrated, sensors 72 and 70 may monitor the positive electrolyte and negative electrolyte conditions at the positive electrode compartment 22 and the negative electrode compartment 20, respectively. Sensors 72 and 70 may be pH probes, optical probes, pressure sensors, voltage sensors, etc. It should be appreciated that sensors may be positioned at other locations throughout the redox flow battery system 10 to monitor electrolyte chemistry and other properties.

[0043] For example, a sensor may be positioned in an external acid tank (not shown) to monitor the acid volume or pH of the external acid tank, wherein the acid from the external acid tank may be supplied to the redox flow battery system 10 via an external pump (not shown) to reduce the formation of precipitates in the electrolyte. Additional external tanks and sensors may be installed for supplying 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 transmit signals to the controller 88. In addition, during the hydration process of the redox flow battery system 10, the controller 88 may send signals to actuators such as valves and pumps of the field hydration system. For example, sensor information may be transmitted to the controller 88, which in turn may actuate pumps 30 and 32 to control the flow of electrolyte through the redox flow battery unit 18, or perform other control functions. In this way, the controller 88 may respond to one or a combination of sensors and detectors.

[0044] The redox flow battery system 10 may further include a H2 gas source. In one example, the H2 gas source may include a separate dedicated hydrogen gas storage tank. Figure 1 In an example of the present invention, H2 gas may be stored in and supplied from the integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supply additional H2 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 H2 gas to the inlets of the electrolyte rebalance reactors 80 and 82. For example, a mass flow meter or another flow control device (which may be controlled by the controller 88) may regulate the flow of H2 gas from the integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supplement the H2 gas generated in the redox flow battery system 10. For example, when a gas leak is detected in the redox flow battery system 10 or the reduction reaction rate is too low at a low hydrogen partial pressure, H2 gas may be supplied from the integrated multi-chamber electrolyte storage tank 110 to rebalance the SOC of the electroactive materials in the positive electrolyte and the negative electrolyte. For example, the controller 88 may supply H 2 gas from the integrated multi-chamber electrolyte tank 110 in response to a measured pH change or in response to a measured SOC change of the electrolyte or electroactive material.

[0045] For example, an increase in pH of the negative electrolyte chamber 50 or the negative electrode compartment 20 may indicate that H2 gas is leaking from the redox flow battery system 10 and / or the reaction rate is too slow at the available hydrogen partial pressure, and the controller 88, in response to the pH increase, may increase the supply of H2 gas from the integrated multi-chamber electrolyte tank 110 to the redox flow battery system 10. As another example, the controller 88 may supply H2 gas from the integrated multi-chamber electrolyte tank 110 in response to a pH change, wherein 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 gas to increase the Fe 3+ The reduction rate of ions and the generation rate of protons can be increased, thereby reducing the pH of the positive electrolyte. In addition, the pH of the negative electrolyte can be adjusted by exchanging Fe from the positive electrolyte to the negative electrolyte. 3+ The hydrogen reduction of ions is reduced by protons generated on the positive electrode side and exchanged to the negative electrode electrolyte due to the proton concentration gradient and electrophoretic force. In this way, the pH of the negative electrode electrolyte can be maintained in a stable region while reducing the Fe (exchanged from above the positive electrode compartment 22). 3+ Risk of ion precipitation as Fe(OH)3.

[0046] Other control schemes may be implemented to control the supply rate of H2 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 an optical sensor. In addition, the pH or SOC change that triggers the action of the controller 88 may be based on a rate of change or change measured over a certain period of time. The time period of the rate of change may 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 time period may be reduced, and since the time constant may be small, local changes in concentration (e.g., due to side reactions or gas leaks) may be measured quickly.

[0047] The controller 88 may further execute 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 so as to cause iron pre-formation at the negative electrode 26 during system conditioning (wherein the system conditioning may include an operating mode 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 electroplate iron metal on the negative electrode 26 to improve the battery charge capacity during subsequent battery cycling (thus, iron metal may be pre-formed for battery cycling). The controller 88 may further perform the electrolyte rebalancing discussed above to allow the redox flow battery system 10 to remove excess hydrogen and reduce Fe 3+ion concentration. In this way, pre-forming iron at the negative electrode 26 and running electrolyte rebalancing during system conditioning can increase the overall capacity of the redox flow battery cell 18 during battery cycling by mitigating iron plating losses. As used herein, battery cycling (also referred to as a "charge cycle") can include alternation between a charge mode and a discharge mode of the redox flow battery system 10.

[0048] It should be understood that all components except the sensors 60 and 62 and the integrated multi-chamber electrolyte tank 110 (and the components contained therein) can be considered to be contained in the power module 120. Therefore, the redox flow battery system 10 can be described as including a power module 120 that is fluidically coupled to the integrated multi-chamber electrolyte tank 110 and communicatively coupled to the sensors 60 and 62. In some examples, each of the power module 120 and the multi-chamber electrolyte tank 110 can be contained in a single housing (not shown) so that the redox flow battery system 10 can be housed in a single location as a single unit. It should also be understood that the positive electrolyte, the negative electrolyte, the sensors 60 and 62, the electrolyte rebalance reactors 80 and 82, and the integrated multi-chamber electrolyte tank 110 (and the components contained therein) can be considered to be contained in the electrolyte subsystem 130. Therefore, the electrolyte subsystem 130 can supply one or more electrolytes to the redox flow battery cell 18 (and the components contained therein).

[0049] The size (e.g., volume) of an integrated multi-chamber electrolyte tank (e.g., integrated multi-chamber electrolyte tank 110) can determine the capacity of a 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 tank. Without increasing the size of the electrolyte tank, the capacity of the battery can be increased by increasing the concentration of iron salts in the electrolyte. By increasing the capacity without changing the size of the electrolyte tank, the total energy density of the system can be increased. As described below, high energy density electrolytes can allow iron ion concentrations up to 3M by adding supporting salts such as ammonium salts and calcium salts. Typically, potassium chloride can be used as a supporting salt, and increasing the iron salt concentration may require increasing the potassium chloride concentration at the same time. However, when the potassium chloride concentration increases with the increase in the iron salt concentration, potassium chloride may not remain completely soluble and / or stable. Minimizing the amount of potassium chloride while increasing the concentration of alternative supporting salts (e.g., ammonium chloride and calcium chloride) can increase the solubility of the iron salt while maintaining the supporting salt completely soluble in the electrolyte.

[0050] Reference now Figure 2, shows a graph 200 in which the discharge capacity as a percentage of the theoretical discharge capacity is plotted as a function of the number of cycles of the IFB cycle between two different SOCs (e.g., 20% to 80%). Arrow 208 indicates the direction of increasing the discharge capacity along the y-axis of the graph 200, and arrow 210 indicates the direction of increasing the number of cycles along the x-axis of the graph 200. Trace 202 corresponds to an IFB configured 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 additional supporting salts, such as ammonium salts or calcium salts. In this way, comparing an IFB configured with a reference electrolyte to an IFB configured with a high energy density electrolyte can highlight the differences caused by the addition of ammonium salts or calcium salts. Trace 204 corresponds to an IFB configured with a first high energy density electrolyte. Trace 206 corresponds to an IFB configured with a second high energy density electrolyte. Table 1 below shows the composition of the reference electrolyte, as well as the composition range that can be used for high energy density electrolytes (e.g., a first high energy density electrolyte and a second high energy density electrolyte).

[0051] Table 1. Concentrations of reference electrolytes and high energy density electrolytes

[0052]

[0053] The high energy density electrolyte may include an iron salt, such as FeCl2, in a concentration ranging 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 the increased iron salt concentration, the high energy density may include ammonium chloride (NH4Cl) and optionally calcium chloride (CaCl2) to provide additional supporting salts. The content of potassium chloride (KCl) may be as high as 1.5M and may not exceed 1.5M. In addition, although the above table lists chloride salts, other anions (such as SO4 2- etc.) are also considered to be within the scope of this application. Figure 2 The traces shown in Figure 4 to 1 Those traces included in the graph shown in Figure 0 can be collected from an IFB containing a first high energy density electrolyte or a second high energy density electrolyte. The second high energy density electrolyte differs from the first high energy density electrolyte in that calcium chloride is additionally included. Table 2 below lists the component concentrations of the first high energy density electrolyte and the second high energy density electrolyte.

[0054] Table 2. Composition of the first high energy density electrolyte and the second high energy density electrolyte.

[0055]

[0056] Figure 2Trace 202 shows a decrease in the discharge capacity of an IFB configured with a reference electrolyte after the second cycle. The discharge capacity continues to decrease in subsequent cycles, dropping to 50% discharge of the theoretical discharge capacity at the completion of 5 cycles. In contrast, traces 204 and 206 show that when the IFB is configured with a first high energy density electrolyte or a second high energy density electrolyte, the initial discharge capacity of the system is maintained at 100% of the theoretical discharge capacity for at least 5 cycles. Traces 202, 204, and 206 all correspond to systems using an electrolyte with 2.5M FeCl2. It can be seen that the addition of ammonium chloride and optional calcium chloride to the high energy density electrolyte helps maintain the discharge capacity of an IFB with a high iron salt concentration over multiple cycles. Since the electrolyte salt containing both iron and potassium salts remains completely dissolved for at least 5 cycles, the discharge capacity of the IFB can be maintained.

[0057] Now turn to Figure 3 , graph 300 shows the cell voltage (e.g., potential) of an IFB configured with a reference electrolyte as a function of time. Arrow 308 indicates the direction of increasing cell voltage along the y-axis of graph 300, and arrow 310 indicates the direction of increasing elapsed time along the x-axis of graph 300. During the course of the experiment (e.g., increasing time), the IFB may be cycled continuously from 20% SOC to 80% SOC multiple times. For example, the time period indicated by bracket 304 may correspond to an increase in the IFB system from 20% SOC to 80% SOC, and the time period indicated by bracket 306 may correspond to a decrease in the IFB from 80% SOC to 20% SOC. Trace 302 may correspond to the potential measured during the charge / discharge of the IFB system, and indicates unstable performance. The maximum charged cell voltage reached at 80% SOC may drop during multiple charge / discharge cycles, which may indicate electrolyte degradation.

[0058] As mentioned above about Figure 2 As discussed, when the concentration of iron salt increases beyond 2M without the addition of supporting salts such as ammonium chloride and calcium chloride, the discharge capacity of the IFB may decrease as the IFB system undergoes continued charge and discharge cycles. This can be demonstrated by trace 302, which shows the instability of the cell potential measured during multiple charge / discharge events as described above. This may be due to the insolubility of 2.5M iron in the unsupported electrolyte causing the electrolyte to degrade over time.

[0059] Now turn to Figure 4 , graph 400 shows a graph similar to that discussed above. Figure 3Trace 402 plots data collected in the manner of data collected by the IFB system ... Figure 3 Unlike the data shown in , trace 402 shows a more stable performance of IFB over multiple consecutive charge / discharge cycles. Trace 402 indicates that the cell voltage repeatedly cycles between a more consistent maximum cell voltage and a minimum cell voltage during the six charge / discharge cycles shown compared to the reference electrolyte. Figure 4 The data shown in show that although the first high energy density electrolytes include Figure 3 The results show that the IFB has the same iron salt concentration as the reference electrolyte measured in , but is more resistant to degradation during battery operation and the battery performance is more stable.

[0060] Now turn to Figure 5 , the graph 500 shows a graph of Figure 3 and Figure 4 Trace 402 plots data collected in the manner of data collected by the IFB system ... Figure 4 , trace 502 indicates uniform and reproducible changes between two potential values ​​as the IFB is charged and discharged over the period of time over which the data was collected, indicating more stable battery performance than that achieved with the reference electrolyte.

[0061] Figure 6A graph 600 depicting cycle efficiency as a function of charge / discharge cycle number is shown, the cycle efficiency comprising: coulombic efficiency, voltaic efficiency and energy efficiency. Arrow 610 indicates the direction of increasing efficiency along the y-axis of graph 600, and arrow 612 indicates the direction of increasing cycle number along the x-axis of graph 600. The data shown in graph 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. In addition to the changes between the two early cycles and the last two cycles, the slopes of traces 602, 604 and 608 may be close to zero. In other words, the efficiency of the IFB comprising the first high energy capacity electrolyte may remain relatively uniform over multiple cycles without reducing the charge capacity.

[0062] Now turn to Figure 7 , graph 700 shows cycle efficiency as a function of charge / discharge cycle number, including: coulombic efficiency, voltaic efficiency, and energy efficiency. Arrow 708 indicates the direction of increasing efficiency along the y-axis of graph 700, and arrow 710 indicates the direction of increasing cycle number along the x-axis of graph 700. The data shown in graph 700 may be similar to Figure 6 The data shown in graph 600 of , but collected using an IFB system incorporating the second highest energy density electrolyte. Traces 702, 704, and 706 correspond to energy efficiency, voltaic efficiency, and coulombic efficiency, respectively. Figure 6 602 and 604 and 608, traces 702, 704 and 706 remain relatively flat and indicate the stability of the electrolyte over multiple cycles. In addition, traces 702, 704 and 706 can show continued stability, including until the last cycle of the second highest energy density electrolyte shown on graph 700.

[0063] As mentioned above about Figures 2 to 7 As described above, the first high energy density electrolyte and the second high energy density electrolyte can be more stable than the reference electrolyte over multiple charge / discharge cycles. Although the first high energy density electrolyte and the second high energy density electrolyte are both relatively stable, especially compared to the reference electrolyte, there may also be stability differences between the first embodiment and the second embodiment of the high energy density electrolyte, which can be Figures 8 to 9 Understand.

[0064] Now turn to Figure 8 , graph 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 graph 800, and arrow 808 shows the direction of increasing cycle number along the x-axis of graph 800. Trace 804 corresponds to the first high energy density electrolyte and can be compared to Figure 2 Trace 802 corresponds to the second highest energy density electrolyte and can be compared to Figure 2 206. However, traces 802 and 804 are plotted in graph 800 for 16 cycles, as opposed to the 5 cycles plotted in graph 200. Trace 804 maintains a stable discharge capacity for 4 cycles, after which the discharge capacity fluctuates. Trace 802 maintains a stable discharge capacity for at least 10 cycles, after which the discharge capacity fluctuates. As demonstrated by the comparison between traces 802 and 804, the calcium chloride included in the second embodiment of the high energy density electrolyte can maintain the discharge capacity of the IFB for a greater number of cycles.

[0065] Now turn to Fig. 9 , graph 900 shows the potential as a function of time. Arrow 906 indicates the direction of increasing cell voltage along the y-axis of graph 900, and arrow 908 indicates the direction of increasing elapsed time along the x-axis of graph 900. Trace 902 corresponds to the first high energy density electrolyte and can be compared to Figure 4 Trace 904 corresponds to the second highest energy density electrolyte and can be compared to Figure 5 Trace 902 and trace 904 may be substantially the same, indicating that adding calcium chloride to a high energy density electrolyte may have little effect on the potential of the IFB at low charge density.

[0066] In this way, in addition to supporting salts including calcium salts and ammonium salts, high energy density electrolytes with iron concentrations up to 3M can increase the capacity of the IFB system without increasing the electrolyte volume. Therefore, the footprint of the IFB system can remain compact. Adding supplementary supporting salts such as calcium salts and ammonium salts allows the total iron concentration of the IFB electrolyte to be increased without adversely affecting battery performance. Ammonium salts and / or calcium salts can maintain the desired low cost and low toxicity characteristics of the IFB electrolyte. In addition, ammonium salts and calcium salts can be added to the negative electrode electrolyte in the negative electrode compartment of the IFB and the positive electrode electrolyte in the positive electrode compartment of the IFB, wherein the negative electrode electrolyte is adjusted to have a lower pH than the positive electrode electrolyte. Maintaining a shared composition of the two electrolytes can minimize the impact of ion exchange.

[0067] The present disclosure also provides a support for the electrolyte of a redox flow battery, including: a redox active material dissolved in the electrolyte and having a concentration greater than 2.0M, and a plurality of dissolved supporting salts, including potassium salts, ammonium salts, calcium salts, and manganese salts. In the first example of the system, the redox active material is an iron salt. In the second example of the system, the first example is optionally included, and the redox active material has a concentration range of 2.0M to 3.0M. In the third example of the system, one or both of the first and second examples are optionally included, and the potassium salt is potassium chloride with a concentration range of 0.5M to 1.5M. In the fourth example of the system, one or more or each of the first to third examples is optionally included, and the ammonium salt is ammonium chloride with a concentration range of 0.5M to 2.5M. In the fifth example of the system, one or more or each of the first to fourth examples is optionally included, and the calcium salt is calcium chloride with a concentration range of 0M to 1.5M. In a sixth example of the system, optionally including one or more or each of the first to fifth examples, the manganese salt is manganese chloride with a concentration range of 0 M to 0.25 M. In a seventh example of the system, optionally including one or more or each of the first to sixth examples, the system further includes: boric acid with a concentration range of 0 M to 0.25 M. In an eighth example of the system, optionally including one or more or each of the first to seventh examples, the electrolyte is an aqueous electrolyte.

[0068] The present disclosure also provides a support for a redox flow battery, comprising: an electrolyte solution, wherein the composition of the electrolyte solution is shared between an electrolyte in a negative electrode compartment and an electrolyte in a positive electrode compartment, the electrolyte solution comprising dissolved iron and a dissolved supporting salt comprising an ammonium salt at a concentration of at least 2.0 M. In a first example of the system, the electrolyte solution in the negative electrode compartment has a lower pH than the electrolyte solution in the positive electrode compartment, and wherein the dissolved iron and the dissolved supporting salt remain completely dissolved in both the negative electrode compartment and the positive electrode compartment. In a second example of the system, optionally including the first example, the dissolved supporting salt further includes a calcium salt, and when the dissolved supporting salt includes a calcium salt, the discharge capacity of the redox flow battery is more uniform over multiple cycles than when the solution does not include a calcium salt. In a third example of the system, optionally including one or both of the first example and the second example, the dissolved supporting salt further includes a calcium salt, and when the dissolved supporting salt includes a calcium salt and an ammonium salt, the discharge capacity of the redox flow battery is more uniform across multiple cycles than when the dissolved supporting salt does not include a calcium salt or an ammonium salt. In a fourth example of the system, optionally including one or more or each of the first to third examples, the dissolved supporting salt further includes a calcium salt, and the discharge capacity of the redox flow battery remains stable for at least 10 cycles. In a fifth example of the system, optionally including one or more or each of the first to fourth examples, the dissolved supporting salt further includes potassium chloride and manganese chloride.

[0069] The present disclosure also provides a support for an electrolyte composition for a redox flow battery, the electrolyte composition comprising: an iron salt at a concentration of at least 2.0M, 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. In a first example of the system, the iron salt participates in a redox reaction during the operation of the redox flow battery, and wherein the ammonium salt and the additional supporting salt do not participate in the redox reaction during the operation of the redox flow battery. In a second example of the system, optionally including the first example, when the ammonium salt is not present in the electrolyte composition, the maximum charging battery voltage of the redox flow battery decreases with continued cycling. In a third example of the system, optionally including one or both of the first example and the second example, the additional supporting salt includes a calcium salt at a concentration of up to 1.5M. In a fourth example of the system, optionally including one or more or each of the first to third examples, the redox flow battery is an iron redox flow battery.

[0070] The appended claims specifically point out certain combinations and sub-combinations that are considered novel and non-obvious. These claims may refer to "an" element or a "first" element or the equivalent thereof. Such claims should be understood to include the incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements and / or properties may be claimed by amendment of the claims of the present invention or by the presentation of new claims in this or a related application. Such claims, whether broader, narrower, equivalent or different in scope than the original claims, are also deemed to be included in the subject matter of the present disclosure.

Claims

1. An electrolyte for a redox flow battery, comprising: a redox active material dissolved in the electrolyte and having a concentration greater than 2.0 M; as well as A variety of dissolved supporting salts including potassium, ammonium, calcium and manganese salts.

2. The electrolyte according to claim 1, wherein the redox active material is an iron salt.

3. The electrolyte of claim 1, wherein the concentration of the redox active material ranges from 2.0 M to 3.0 M.

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

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

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

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

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

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

10. A redox flow battery comprising: An electrolyte solution, wherein the composition of the electrolyte solution is shared between an electrolyte in a negative electrode compartment and an electrolyte in a positive electrode 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. The redox flow battery of claim 10, wherein the electrolyte solution in the negative electrode compartment has a lower pH than the electrolyte solution in the positive electrode compartment, and wherein the dissolved iron and the dissolved supporting salt remain completely dissolved in both the negative electrode compartment and the positive electrode compartment.

12. 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 include the calcium salt.

13. 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 across multiple cycles than when the dissolved supporting salt does not comprise the calcium salt or the ammonium salt.

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

15. 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; Ammonium salts at concentrations ranging from 0.5 M to 2.5 M; as well as Additional supporting salt at a concentration of 2.5 M or less.

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

18. The electrolyte composition of claim 16, wherein when the ammonium salt is absent from the electrolyte composition, the maximum charge cell voltage of the redox flow battery decreases with continued cycling.

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

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

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