WO2011149624A1 - Secondary battery system - Google Patents

Abstract

Secondary batteries, such as flow batteries, and compositions for use in such batteries are provided. The secondary battery systems described herein are designed for charge and discharge with very small capacity loss per cycle over a long period of time.

Description

SECONDARY BATTERY SYSTEM CROSS REFERENCE TO RELATED APPLICATION [0001] This application claims benefit of priority to United States Provisional

Application Serial No. 61/347,753, filed May 24, 2010, entitled "SECONDARY

BATTERY SYSTEM", which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The invention relates to secondary battery systems, such as flow battery systems, in which the electrochemically active species remain chemically stable and in which the battery retains capacity over a long time period.

BACKGROUND

[0003] When a hydrocarbon is used as an energy source, it is typically thermally oxidized and generates a gas at a high pressure, which does work in a machine to create power. This indirect method is fundamentally limited by the minimum and maximum temperatures associated with the process defined by the Carnot Efficiency, practically less than 30%. There are ways to directly convert heat to electricity without the use of a mechanical machine, but they are not yet practical (e.g., thermoelectric or Seebeck Effect, or reverse Peltier Effect). These methods are still subject to the Carnot Efficiency due to the requirement of creating heat. Even nuclear energy falls prey to the requirements of a Carnot Efficiency.

[0004] In contrast, a battery can directly convert chemical energy to electrical energy, so it provides a theoretical advantage in avoiding the significant losses associated with the Carnot Efficiency. However, there is a small loss associated with electrochemical energy conversion, because in most cases a small amount of heat is generated that is related to the change in entropy of the system due to the electrode reactions.

[0005] A battery fundamentally allows a chemical reaction to proceed without significant production of heat by separating the components in such a manner that electrons are restricted to an external circuit where they can do work, while the chemical components react in a manner that avoids direct contact between the reactants. Typically, the two electrodes are the reactants and the electrolyte prevents direct contact between the electrodes, while providing a conductive, ionic path. As long as the electrolyte does not provide any electronic conductive path (i.e., electrons or holes), the two electrodes will generally not react without current flowing through the external circuit.

[0006] For a significant number of applications, it is satisfactory for a battery to yield a fixed amount of energy and power. It may be used and then disposed of. Single use batteries are referred to as "primary" batteries and include common alkaline cells. An advantage of primary batteries is that nothing else is required, since all the energy is contained in a freestanding device.

[0007] Many other applications require significantly more energy than can be supplied by a primary battery or would benefit from a more long term economical solution. One such solution is a "secondary" battery, one that can be discharged and then electrically recharged. This type of battery must be associated with a source of electrical energy for the charging process. Conservation of materials and resources is a theoretical advantage of a secondary cell over a primary cell. However, the technical requirements of a secondary cell are significantly more complex than those of a primary battery.

[0008] In the search to replace fossil fuels as the primary energy source for vehicles, electrochemical energy conversion is a prime candidate. Energy can theoretically be produced from a number of more efficient sources (e.g., large power plants, solar, wind) and stored chemically at a high energy and power density. For vehicle applications, one option is a secondary battery that can be discharged and then electrically charged when not in use or by recycling a fraction of the kinetic energy from a source such as braking.

Another practical option is a rechargeable primary battery, e.g. , a zinc-air battery. A rechargeable primary battery discharges and the products of the electrochemical reaction are contained in the electrolyte, which can be removed and replaced at an "electrolyte station." The electrodes are inert and thus are not consumed or modified during the discharge, so they may be used many times. Frequently, an air (oxygen) electrode is employed, which has the advantage of not requiring one of the chemical components to be carried within the battery. Primary batteries are fundamentally less difficult to design than secondary batteries and provide a much wider chemical pallet to choose from.

[0009] A large development effort in the secondary battery field has focused on the use of lithium. Due to the properties of lithium, such a battery has advantages of high energy and low weight. However, rechargeable primary batteries may also meet the demands for some potential applications with less energy requirements or those with intermittent energy requirements.

[0010] A "traditional" battery contains two electrodes and an electrolyte.

Electrochemically active components are contained in the two electrodes, while the electrolyte does not take place in the cell reaction and is not fundamentally changed due to the electrochemical reactions. The mass of the electrodes dictates the quantity of energy in the battery. The geometric design and surface area of the electrodes dictate the available power. Typically, the electrolyte is only required to prevent the direct contact of the two reactants, so it can be minimized in volume (which also aids in keeping the cell resistance low). The electrolyte typically has ionic components that are common to both electrodes. This type of battery can be of primary or secondary design.

[0011] Another type of battery is a secondary lead acid battery. It is unique in its operation and thus is not a "traditional" battery. Due to its design, the main component in both electrodes (lead in lead and lead in lead oxide) is not a component in the electrolyte. Additionally, a component of the electrolyte (sulfate anions) is a fundamental component of both electrodes (lead sulfate). What this means is that the quantity of electrolyte is tied to the energy capacity of the battery along with the size of both electrodes.

[0012] A fuel cell is fundamentally a primary battery. A fuel cell has two gaseous reactants that can be continuously supplied, typically hydrogen and oxygen gas. This can include hydrogen or hydrocarbons for the anode reaction and typically air or oxygen for the cathode reaction. Electrolytes are typically not consumed in the discharge of the fuel cell. Nomenclature can be quite confusing in the literature. A cell with one liquid or solid electrode and one gas electrode is often called a fuel cell (such as the rechargeable primary cells discussed previously). Generally, when a liquid containing the active species is substituted for hydrogen, an ionic conducting membrane must be used to prevent direct mixing of this liquid with the reactant and product of the oxygen electrode.

[0013] A secondary "flow" battery differs from a traditional secondary battery in several ways. A flow battery contains two compartments, with one compartment containing an anode in contact with a liquid composition containing electroactive species (the anolyte) and the other compartment containing a cathode in contact with a liquid composition containing electroactive species (the catholyte). The combination of the anode and the anolyte is referred to as the anode half cell and the combination of the cathode and the catholyte is referred to as the catholyte half cell. The physical locations of the anode and cathode half cells at any particular point in time are dependent on whether the battery is in the process of being charged or discharged. In other words, the anode half cell during charging of the battery will be the cathode half cell during discharge, and the cathode half cell during charging of the battery will be the anode half cell during discharge. The reactants are included in the anolyte and catholyte and not the electrodes. The electrodes are typically inert and do not take place in the half cell reactions. One tank will be the anolyte tank during charging of the battery and the catholyte tank during discharging. The other tank will be the catholyte tank during charging and the anolyte tank during

discharging.

[0014] The reactants in the anolyte and catholyte must be separated from one another to avoid a direct chemical reaction in the battery. This is often done with an ion conducting membrane which allows only cations or only anions to migrate between compartments without allowing the anolyte and catholyte to chemically mix. When electrons travel in the external circuit, ions must move across the ion conducting membrane to maintain charge balance. The membrane is essentially a third electrolyte in series with the anolyte and catholyte. Another option is to use a separator instead of an ion exchange membrane. A separator provides a physical barrier between the anolyte and catholyte allowing some degree of interdiffusion of the two electrolytes but minimizes any significant convective mixing of the two liquids. The advantages include lower costs, potentially a higher degree of chemical stability and a low contribution to resistance. Disadvantages are an enhanced self discharge rate and the potential for all species to mix among the two liquids. Generally, the separator will not be a barrier to specific ions. This becomes significant when either half cell reaction includes protons or hydroxyl ions and the pH varies with the state of charge or discharge of the cell.

[0015] In a redox flow battery, plating reactions are avoided, and the oxidation and reduction reactions are limited to changes in the oxidation state of the soluble reactants and products in the anolyte and catholyte. Typically, the process of plating a metal in a battery is associated with some degree of capacity loss due to the nature of the plating process which can "lose" some of the plated metal if it falls from the electrode. A huge industry exists trying to find novel materials, cell configurations and processes which can solve the fundamental problem of forming and dissolving solids from a liquid (or solid) electrolyte without incurring capacity loss. A redox flow battery avoids this problem.

[0016] In a redox flow battery, the geometry of the electrodes and number of cells are dictated by the power requirements, while the quantity of liquid containing electroactive species is dictated by the energy requirements. Cells can be combined in series or parallel, or in a series/parallel arrangement to achieve the desired current and voltage for discharging and charging. The anolyte and catholyte may be stored remotely and pumped to the cells in the system when discharging or charging. Theoretically at least, this allows a flow battery system to be optimized for its specific application. With a traditional battery, either the energy or the power will dictate the size of the electrodes. The geometry can be

manipulated to attempt to optimize the design. With a traditional battery with a given power requirement, the size of the electrodes and the cell to contain the electrodes are linearly related to the energy requirements. A large part of the cost of such a battery will typically increase with the energy requirements. In contrast, with a flow battery, the quantity of anolyte plus catholyte is proportional to the energy requirements. Only the anolyte plus catholyte and its storage requirements need to be scaled with energy.

[0017] One variation of a flow battery is to have one oxygen electrode and one redox electrode. Oxygen is consumed during the discharge cycle and liberated during the charging cycle. Although the oxygen in the air is supplied for "free," the oxygen reaction consumes protons in an acidic electrolyte or generates hydroxyl ions in a basic electrolyte to complete the cell discharge reaction. Therefore, an electrolyte is still required with a typical ion exchange membrane. Acid and bases can typically exist at very high concentrations (10-30M) for relatively high energy densities, in comparison with typical flow cell reactant concentrations (2-5M) which are often limited by their solubility. In practical terms, however, the acid or base concentration is often limited to about 3M by the properties of the ion conducting membrane. Another practical issue with an oxygen electrode with a basic electrolyte is the reaction with carbon dioxide that naturally occurs in air, in which the carbon dioxide concentration is about 400 ppm. Carbonate ions may be produced which may not be adequately soluble in some electrolyte compositions and may precipitate in the oxygen electrode and ion exchange membrane. A potential remedy is to remove the carbon dioxide from the air, but this involves additional expense. In some fuel cell applications, pure oxygen is supplied to the electrode rather than air, which serves the purpose of avoiding this problem. An acidic oxygen electrode avoids the carbon dioxide problem but often suffers from relatively slow reaction kinetics. This may conflict with the power requirements of a flow battery. An oxygen electrode may also require an expensive electrode material to reduce the overpotential associated with oxygen reduction or generation.

[0018] Liquid discharge anolytes have previously been used with oxygen electrodes.

Typically, the product of the discharge anode reaction with such an anolyte includes a gaseous species such as carbon dioxide, which is not advantageous for secondary cell applications.

[0019] Hybrid flow batteries contain one electrode that plates and dissolves a material (typically a metal) and one redox electrode. Typically at the metal electrode, a cation is reduced or the metal is oxidized to a cation, while the other electrode is an inert electrode in an electrolyte with the active redox species. A membrane or separator must still separate the two electrodes to avoid the redox electrolyte from contacting the active metal electrode.

[0020] One type of half cell reaction that can be incorporated into a flow battery is a simple redox reaction involving a single ionic species, which is reduced and oxidized to different oxidation states. Examples include metals such as iron, chromium and vanadium. Another type of half cell reaction involves an ion (typically an anion) that changes oxidation state and changes its chemical formula. Such a half cell reaction also includes either protons or hydroxyl ions and water. This type of half cell reaction often exhibits inferior reaction kinetics in comparison to a single ionic species, due to the rearrangement of atoms that is required. However, such a reaction may have other desirable features, such as requiring a multitude of electrons to complete the reaction.

[0021] Another important consideration is that the pH of the anolyte and catholyte may vary as the half cell reaction proceeds. Depending on the overall cell reaction, the pH of the anolyte and catholyte may stay relatively constant or it may vary over a charge / discharge cycle. The maximum energy density of the cell may be dependent on the state of charge with the highest proton or highest hydroxide concentration due to the stability of the membrane, rather than by the solubility of the active species.

[0022] There is a need for an improved secondary flow battery, with features such as. low cost, long lifetime (e.g., 5 years with little or no maintenance and 10,000 cycle life) , minimal capacity loss over lifetime, high energy (e.g., 50 KW-HR to IMW-HR), and high power (e.g., discharge all of the energy in 5 hours (energy (KW-Hr)/5 (Hr)).

[0023] Desirable features of a flow battery system include:

- delivery of energy and power at a reasonable cost

- high energy density (energy/unit volume or energy/unit mass)

- high power density (power/unit volume or power/unit mass)

-chemical stability of all of the species present in the anolyte and catholyte over the lifetime of the battery

-maintains its capacity over the lifetime of the battery

-high coulombic, voltaic and energy efficiencies

-minimal hydrogen gas evolution (degrades capacity, consumes water and is potentially explosive)

-minimal oxygen gas evolution (degrades capacity and consumes water)

-fast reaction kinetics (allows adequate power without significant energy losses)

-minimal corrosion (appropriate choice of materials for a given system)

-long electrode and membrane lifetime (appropriate choices for a given system)

BRIEF SUMMARY OF THE INVENTION

[0024] In one aspect, a secondary battery is provided. The secondary battery contains two compartments, one compartment containing an anolyte and an anode and another compartment containing a catholyte and a cathode. The two compartments are divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte and permits an ionically conducting path between the anode and cathode. The battery includes an overall cell reaction that includes an anode half cell reaction and a cathode half cell reaction. Prior to charging, the anolyte and the catholyte contain substantially the same liquid composition that predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction. In some embodiments, the liquid composition is at a basic pH, for example, pH about 10 to about 15, about 10 to about 13, or about 12 to about 15. In other embodiments, the liquid

composition is at an acidic pH, for example, pH about -1 to about 4, about -1 to about 2, or pH about 1 to about 4. Upon charging the secondary battery, (i) in the charge anolyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and (ii) in the charge catholyte, the product of the discharge anode reaction is reduced to a lower oxidation state, and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. Upon discharging the secondary battery, (i) in the discharge catholyte, the product of the discharge cathode reaction is in its lowest oxidation state or the lowest oxidation state achieved during the charging and discharging of the cell, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and (ii) in the discharge anolyte, the product of the discharge anode reaction is in its highest oxidation state or the highest oxidation state achieved during the charging and discharging of the cell and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. Optionally, after fully, substantially fully, or partially discharging the secondary battery, the anolyte and catholyte are mixed to restore the initial capacity of the system and the mixed composition predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction. In some embodiments, the secondary battery is a flow battery.

[0025] In some embodiments, the liquid composition containing electroactive species is aqueous. In some embodiments, the two compartments of the battery are separated by an ion exchange membrane. In some embodiments, the anolyte and catholyte are mixed after each discharge cycle. In some embodiments, the anolyte and catholyte are mixed periodically prior to charging the battery.

[0026] In some embodiments, the discharge cathode half cell reaction includes reduction of a halogen oxyanion. In some embodiments, the halogen is bromine, chlorine, iodine, or a mixture thereof. In some embodiments, the halogen oxyanion is bromate, perbromate, hypobromite (e.g., in acid), or alkaline bromine water (BrO ) (e.g., in base). In some embodiments, the liquid composition containing electroactive species, i.e., the anolyte and catholyte, contains a complexing agent, for example, in a composition that contains a halogen-containing electroactive species, i.e., bromine, chlorine, iodine, or a mixture thereof. For example, in one embodiment in which the half cell reaction includes bromide, a complexing agent may form a complex with bromine to form a second phase with a density greater than the anolyte or catholyte. This may minimize bromine from escaping as a gas.

[0027] In some embodiments, the discharge cathode half cell reaction includes reduction of a metal oxyanion, for example, a manganese or chromium oxyanion.

[0028] In some embodiments, the discharge anode half cell reaction includes oxidation of phosphite, hypophosphite, phosphorous acid, hypophosphorous acid, or a sulfur containing oxyanion (e.g., thiosulfate). In some embodiments, the discharge anode half cell reaction includes oxidation of a metal oxyanion, for example, a tin oxyanion.

[0029] In another aspect, a secondary battery is provided. The secondary battery contains two compartments, one compartment containing an anolyte and an anode and another compartment containing a catholyte and a cathode. The two compartments are divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte and permits an ionically conducting path between the anode and cathode. The battery includes an overall cell reaction that includes an anode half cell reaction and a cathode half cell reaction, and the discharge anode half cell reaction includes

phosphate/phosphite, phosphate/hypophosphite, formate/carbonate, or sulfur

oxyanion/sulfur oxyanion, e.g., thiosulfate/sulfate.

[0030] In some embodiments, prior to charging, the anolyte and the catholyte contain substantially the same composition. In some embodiments, the secondary battery is a flow battery. In some embodiments, the discharge catholyte and anolyte are mixed prior to charging the battery such that the anolyte and catholyte comprise substantially the same composition prior to charging.

[0031] In some embodiments, prior to charging the secondary battery, the anolyte and the catholyte contain substantially the same liquid composition that predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction, and upon charging the secondary battery, (i) in the charge anolyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and (ii) in the charge catholyte, the product of the discharge anode reaction is reduced to a lower oxidation state, and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. Upon discharging the secondary battery, (i) in the discharge catholyte, the product of the discharge cathode reaction is in its lowest oxidation state or the lowest oxidation state achieved during the charging and discharging of the cell, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and (ii) in the discharge anolyte, the product of the discharge anode reaction is in its highest oxidation state or the highest oxidation state achieved during the charging and discharging of the cell and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. Optionally, after fully, substantially fully, or partially discharging the secondary battery, the anolyte and catholyte are mixed to restore the initial capacity of the system and the mixed composition predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction.

[0032] In another aspect, a secondary battery is provided. The secondary battery contains two compartments, one compartment containing an anolyte and an anode and another compartment containing a catholyte and a cathode. The two compartments are divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte and permits an ionically conducting path between the anode and cathode. The battery includes an overall cell reaction that includes an anode half cell reaction and a cathode half cell reaction, wherein the anode half cell reaction is selected from

phosphate/phosphite, phosphate/hypophosphite, sulfur oxyanion/sulfur oxyanion (e.g., thiosulfate/sulfate), formate/carbonate, and a metal ion (e.g., tin cation), and wherein the cathode half cell reaction is selected from halogen oxyanion/halide, Mn0₄7Mn²⁺, and HCr0₄7Cr²⁺ . In some embodiments, the anolyte and catholyte contain substantially the same composition. In some embodiments, the halogen is bromine, chlorine, iodine, or a mixture thereof. In some embodiments, the halogen oxyanion is bromate or perbromate and the halide is bromide. In some embodiments, a complexing agent is included in the liquid composition containing electroactive species, i.e., the anolyte and catholyte. For example, in some embodiments in which the half cell reaction includes bromide, a complexing agent may form a complex with bromine to form a second phase with a density greater than the anolyte or catholyte. This may minimize bromine from escaping as a gas.

[0033] In some embodiments, prior to charging the secondary battery, the anolyte and the catholyte contain substantially the same liquid composition that predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction, and upon charging the secondary battery, (i) in the charge anolyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and (ii) in the charge catholyte, the product of the discharge anode reaction is reduced to a lower oxidation state, and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. Upon discharging the secondary battery, (i) in the discharge catholyte, the product of the discharge cathode reaction is in its lowest oxidation state or the lowest oxidation state achieved during the charging and discharging of the cell, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and (ii) in the discharge anolyte, the product of the discharge anode reaction is in its highest oxidation state or the highest oxidation state achieved during the charging and discharging of the cell and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. Optionally, after fully, substantially fully, or partially discharging the secondary battery, the anolyte and catholyte are mixed to restore the initial capacity of the system and the mixed composition predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction.

[0034] In any of the secondary battery systems described herein, the product of the discharge cathode half cell reaction in the catholyte includes more than one active species that is oxidized to a higher oxidation state during charging of the battery, and none of the catholyte active species reacts chemically with any other component of the catholyte when the battery is charged or discharged. In another embodiment, the product of the discharge anode half cell reaction in the anolyte includes more than one anolyte active species that is reduced to a lower oxidation state during charging of the battery, and none of the anolyte active species reacts chemically with any other component of the anolyte when the battery is charged or discharged. In a further embodiment, both the products of the discharge anode and cathode half cell reactions include more than one active species and none of the active species in the anolyte and catholyte reacts chemically with any other component of the anolyte or catholyte when the battery is charged or discharged.

[0035] In another aspect, a method for operating a secondary battery, such as a flow battery, containing any of the liquid compositions or half cell reactions described herein, is provided. The secondary battery contains two compartments, each compartment containing an electrode and configured to contain a liquid composition. The two compartments are divided by a membrane or separator that substantially prevents mixing of the liquid in the two compartments and permits an ionically conducting path between the anode and cathode. Prior to charging, a liquid solution is added to the two compartments. The solution is substantially identical in the two compartments and contains the products of the overall cell reaction. In some embodiments, the solution is acidic and in some embodiments, the solution is basic. The method includes charging the battery. When the battery is charged, (i) in the charge anolyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and (ii) in the charge catholyte, the product of the discharge anode reaction is reduced to a lower oxidation state, and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. The method also includes discharging the charged battery. When the battery, is discharged (i) in the discharge catholyte, the product of the discharge cathode reaction is in its lowest oxidation state or the lowest oxidation state achieved during the charging and discharging of the cell, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and (ii) in the discharge anolyte, the product of the discharge anode reaction is in its highest oxidation state or the highest oxidation state achieved during the charging and discharging of the cell and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. The method also optionally includes mixing the discharged anolyte and catholyte after fully, substantially fully, or partially discharging the secondary battery, producing a mixed liquid composition, to restore the initial capacity of the system. The mixed liquid composition predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction. [0036] In another aspect, a liquid composition for use in a secondary battery, such as a flow battery, is provided. In some embodiments, the composition includes a halogen oxyanion as the discharge catholyte active species and phosphite, hypophosphite, sulfur oxyanion (e.g., thiosulfate), formate, or a metal ion (e.g., tin) as the discharge anolyte active species. In some embodiments, the halogen is bromine, chlorine, iodine, or a mixture thereof. In some embodiments, the halogen oxyanion is bromate, perbromate, hypobromite (e.g., in acid), or alkaline bromine water (BrO ) (e.g., in base), or a chlorine or iodine analogue thereof . In some embodiments, the composition includes a complexing agent. In some embodiments, the composition contains a bromate oxyanion as the discharge catholyte active species and a complexing agent. In some embodiments, the composition is aqueous.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 schematically depicts an embodiment of a flow battery in which the anolyte and the catholyte are mixed after discharge and prior to charging the battery.

[0038] Figure 2 schematically depicts an embodiment of a secondary flow battery incorporating a complexing agent, in the discharged state.

[0039] Figure 3 schematically depicts an embodiment of a system for moving oil in a multiple phase flow battery system from the discharge anolyte tank to the discharge catholyte tank.

[0040] Figure 4 schematically depicts an embodiment of a system for moving oil in a multiple phase flow battery system from the discharge anolyte tank to the discharge catholyte tank, with the anolyte and catholyte tanks having different diameters.

[0041] Figure 5 schematically depicts an embodiment of a system for a flow battery with a configuration that allows mixing of the anolyte and catholyte following a full or nearly full discharge.

[0042] Figure 6 schematically depicts an embodiment of a system for a flow battery wherein a pump may be used to transfer oil from the discharge anolyte tank to the discharge catholyte tank.

[0043] Figure 7 schematically depicts an embodiment of a secondary flow battery incorporating a complexing agent, in the fully charged state.

[0044] Figure 8 schematically depicts an embodiment of a system for a flow battery in which catholyte may be pumped from the discharge catholyte tank to avoid the presence of oil at the cathode during discharge.

[0045] Figure 9 schematically depicts an embodiment of a flow battery containing a tin bromine system in the fully discharged state.

[0046] Figure 10 schematically depicts an embodiment of a flow battery containing a charged tin bromine system at the start of the discharge process.

[0047] Figure 11 schematically depicts a cell in which Sn²⁺ is electrochemically converted to Sn⁴⁺. DETAILED DESCRIPTION

[0048] A secondary battery and compositions for use in a secondary battery are provided. In some embodiments, the secondary battery is a flow battery. In some embodiments, the secondary battery described herein is designed to minimize mechanisms that can cause a permanent capacity loss to the battery as it is cycled over a relatively long period of time.

Secondary batteries

[0049] Generally, a secondary battery as described herein contains two

compartments. One compartment contains the anode where oxidation occurs in contact with the anolyte and the other compartment contains the cathode where reduction occurs in contact with the catholyte. The compartments are separated by a membrane or separator to prevent mixing of the anolyte and catholyte during operation of the battery. At discharge, the anolyte and catholyte are substantially identical. After the battery has been charged and discharged, the anolyte and catholyte may optionally be mixed prior to charging the battery again, either after each discharge or periodically after a number of charge/discharge cycles during operation of the battery, such that the anolyte and catholyte are substantially identical prior to charging. The discharge electrochemical reaction that takes place in the anolyte is referred to as the "discharge anode reaction" or "discharge anode half cell reaction," while the electrochemical reaction that takes place in the catholyte is referred to as the "discharge cathode reaction" or the "discharge cathode half cell reaction." The charge electrochemical reaction that takes place in the anolyte is referred to as the "charge anode reaction" or the "charge anode half cell reaction," while the electrochemical reaction that takes place in the catholyte is referred to as the "charge cathode reaction" or the

"charge cathode half cell reaction." The combination of the anode and cathode half cell reactions during charge or discharge is referred to herein as the "overall cell reaction."

[0050] In the secondary battery systems described herein, the electrodes do not take place in the overall cell reaction. The electrodes are inert while the active species are components in the two anolyte and catholyte in the cell. Unlike other types of batteries, plating reactions do not occur in redox flow cells, and the oxidation and reduction reactions are limited to changes in the oxidation states of soluble reactants and products in the anolyte and catholyte. Because the active components are in the anolyte and catholyte, these two liquid solutions must be prevented from direct contact or else a short circuited chemical reaction will occur, which will prevent the extraction of electrical energy from the system. One way of preventing the chemical reaction is to use an ion conducting membrane between the anolyte and catholyte. It is essentially a third electrolyte in series with the anolyte and catholyte. The membrane allows the passage of only specific ions, which maintains charge balance while other ionic species are reacting at the electrodes during the discharge or charge cycle. When electrons travel in the external circuit, ionic species move across the ion conducting membrane in the appropriate direction to maintain charge balance. Similar to other batteries, the three serial electrolytes must allow the conduction of some ionic species while avoiding the conduction of all electronic species across all three serial electrolytes. Without this ionic conduction, an electric field would build up, preventing the flow of current and the extraction of energy. Alternatively, a separator may be used which minimizes mixing of the anolyte and catholyte but still allows contact between the two fluids.

[0051] The proper choice of electrode reactions is critical to the performance of a secondary battery system, such as a redox flow battery system. Undesirable chemical and electrochemical reactions must be avoided to maintain the maximum capacity of the system and minimize self-discharge. Permanent capacity loss is a potentially serious problem when the battery must be operational for many thousands of cycles over many years. The battery systems and compositions described herein may serve to minimize or alleviate these problems.

[0052] In one embodiment of the secondary battery systems described, a single liquid composition is used when the system is first put together. The liquid contains the products of the overall discharge cell reaction. This liquid is then split between two tanks.

The cell is then charged to store energy and discharged to extract the stored energy.

Optionally, after a full or partial discharge, the anolyte and catholyte may be mixed.

Mixing creates the same or substantially the same conditions that preceded the first cycle of the battery, therefore maintaining the same or substantially the same capacity for each subsequent cycle. Mixing may optionally be performed after each charge/discharge cycle, or periodically after a number of charge/discharge cycles. [0053] In some embodiments, the following four considerations with respect to the liquid composition that contains electroactive species serve to minimize self-discharge, optimize the charging efficiency, and/or maintain the capacity of the system over a long lifetime, e.g., many thousands of cycles.

[0054] (I) Start with a single liquid composition that contains the product of the discharge cathode reaction and the product of the discharge anode reaction. Other ionic species may also be present but they should be completely or substantially completely electrochemically inert. To be electrochemically inactive, these species should not or substantially should not change oxidation states during the discharging or charging of the cell. The pH of the solution should be chosen to optimize the performance of the cell.

[0055] In one embodiment, the discharge catholyte products and the discharge anolyte products are stoichiometrically balanced so that if the single starting solution were split into two equal volumes, there would not be an excess of one of the reactants after the cell were fully charged. In other embodiments, the product quantities are not

stoichiometrically balanced. This may be desirable for several reasons, for example, if the quantity of discharge catholyte will not be equal to the quantity of discharge anolyte during operation of the battery, to improve the energy density if one of the species has a much higher solubility compared to other species, if one or more of the discharge catholyte reactants is consumed by a self-discharge chemical reaction at a greater or lesser rate than the discharge anolyte reactants or vice versa, and/or to increase the concentration of one of the products to improve conductivity of the solution.

[0056] (II) After dividing the starting solution into two compartments, one containing the anode and the other containing the cathode, the cell is charged. During charging of the discharge catholyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state. Other species, including, but not limited to, protons or hydroxyl ions, may change in concentration depending on the specific half cell reaction involved. The product of the discharge anolyte reaction must remain unchanged as the discharge catholyte is charged, i.e., it must be electrochemically inactive. This condition will be satisfied if it this species is in its highest oxidation state or the potential of the charge reaction is inadequate to oxidize this species any further. Significant oxygen gas should not be generated during the charging process. Oxygen evolution may be thermodynamically favorable, but the kinetics should not allow significant gas formation. Electrode materials chosen to maximize oxygen gas evolution overpotential will help minimize this undesirable electrochemical oxidation reaction.

[0057] During charging of the discharge anolyte, the product of the discharge anode reaction is reduced to a lower oxidation state. Other species, including, but not limited to, protons or hydroxyl ions, may change in concentration depending on the specific half cell reactions involved. The product of the discharge catholyte reaction must remain unchanged as the discharge anolyte is charged, i.e., it must be electrochemically inactive. This condition will be satisfied if this species is in its lowest oxidation state or the potential of the charge reaction is inadequate to reduce this species any further. Significant hydrogen gas should not be generated during the charging process. Hydrogen evolution may be thermodynamically favorable, but the kinetics should not allow significant gas formation. Electrode materials chosen to maximize hydrogen gas evolution overpotential will help minimize this undesirable electrochemical reduction reaction.

[0058] When the chemical species involved in the half cell reactions are in their proper oxidation states in the anolyte and catholyte, as described above, the starting liquid composition that contains electroactive species will be chemically stable, and there will be no driving force for any further chemical reaction between the components of the solution. Otherwise, in a system that does not satisfy the description above, the electroactive species in the liquid composition will not be in their lowest energy states, and chemical redox reactions will still be possible. This will result in wasted energy during the charging process.

[0059] (III) The charged battery is discharged. In the discharge catholyte, the product of the discharge cathode reaction will be in its lowest oxidation state or the lowest oxidation state achieved during charging and discharging of the cell. The product of the discharge anode reaction is essentially electrochemically inert during the discharge reaction. However, there still may be some quantity of reactant from the discharge cathode reaction if the cell was not completely discharged, if the quantities of electroactive species were not distributed properly before charging or if some self discharge mechanism created some inequity in reactant quantities during the discharge reaction.

[0060] In the discharge anolyte, the product of the discharge anode reaction will be in its highest oxidation state or the highest oxidation state achieved during charging and discharging of the cell. The product of the discharge cathode reaction is essentially electrochemically inert during the discharge reaction. However, there still may be some quantity of reactant from the discharge anode reaction if the cell was not completely discharged, if the quantities of electroactive species were not distributed properly before charging or if some self discharge mechanism created some inequity in reactant quantities during the discharge reaction.

[0061] If the system is acidic and protons are included in the cell reaction, the proton concentration should not drop to too low a value at the start or finish of the discharge or charge reaction.. This is to prevent a significant drop in voltage due to a large proton concentration overpotential or a significant rise in resistance. The pH of the initial single solution should be chosen based on the overall cell reaction and the desired final proton concentration after a full discharge. In some embodiments, the system is acidic and the pH is about -1 to about 4, about -1 to about 2, or about 1 to about 4.

[0062] If the system is basic and hydroxyl ions are included in the cell reaction, the hydroxyl ion concentration should not drop to too low a value at the start or finish of the discharge or charge reaction. This is to prevent a significant drop in voltage due to a large hydroxyl ion concentration overpotential or a significant rise in resistance. The pH of the initial single solution should be chosen based on the overall cell reaction and the desired final hydroxyl concentration after a full discharge. In some embodiments, the system is basic and the pH is about 10 to about 15, about 10 to about 13, or about 12 to about 15.

[0063] (IV) Optionally, after the cell is discharged, the anolyte and catholyte may be mixed. Any residual reactants will chemically react to form the products of the overall cell reaction. Energy will not be usable from this chemical reaction and will result in heat generation. The advantage of mixing is that the initial capacity of the system is restored. After any number of cycles, mixing after a discharge will re-establish the initial system capacity. If I, II and III are followed, the anolyte and catholyte will be very close in composition following a full discharge. Any differences in the two solutions can be attributed to an incomplete discharge or self discharge chemical reactions that occur simultaneously with the desired electrochemical reactions during the charging and discharging processes. Mixing fixes the temporary capacity loss created by these undesirable events. In one embodiment, the anolyte and catholyte are mixed after every charge/discharge cycle. In another embodiment, the anolyte and catholyte are mixed after more than one charge/discharge cycle, for example, after 2, 3, 4, 5, 6, 7, 8, 9, 10, or more cycles. One advantage of periodic mixing compared to mixing after every cycle is that the energy from any partial discharge cycles is not wasted. In one embodiment, the anolyte and catholyte are mixed after a full discharge. In one embodiment, the anolyte and catholyte are mixed after a partial discharge.

[0064] By designing a secondary battery system that follows I, II, and III, only the desired reduction and oxidation reactions will occur at the two electrodes. When charging the cell, only enough energy must be used to convert the active species back to their original states before discharge is initiated plus some extra quantity of energy to convert the species that diffused across the membrane during the operation of the cell. A consequence of this is that at the end of a full discharge cycle, the anolyte and catholyte will be virtually identical for the species that took part in the electrode reactions (including protons or hydroxyl ions). There will be small differences in the concentration of the species in the anolyte and catholyte that diffused across the membrane which will lead to a small capacity loss. In some embodiments, this may cause considerable temporary capacity loss if allowed to build up over many cycles. Optionally, by following IV, mixing the anolyte and catholytes together following a full discharge will restore the full capacity of the system.

[0065] In some embodiments, a secondary battery is provided that contains two compartments, with a first compartment containing an anolyte plus anode and a second compartment containing a catholyte plus cathode, with the first and second compartments divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte and permits an ionically conducting path between the anode and cathode. The battery includes an overall cell reaction that includes an anode half-cell reaction and a cathode half-cell reaction. Prior to charging, the anolyte and the catholyte contain substantially the same liquid composition that predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction. Upon charging the secondary battery, in the charge anolyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged, and in the charge catholyte, the product of the discharge anode reaction is reduced to a lower oxidation state, and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. Upon discharging the secondary battery, in the discharge catholyte, the product of the discharge cathode reaction is in its lowest oxidation state or the lowest oxidation state achieved during the charging and discharging of the cell, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged, and in the discharge anolyte, the product of the discharge anode reaction is in its highest oxidation state or the highest oxidation state achieved during the charging and discharging of the cell and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. Optionally after fully or predominantly discharging the secondary battery, the anolyte and catholyte are mixed to restore the initial capacity of the system and the mixed composition predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction. In some embodiments, the liquid composition that contains electroactive species, i.e., the anolyte and catholyte, is aqueous. In some embodiments, the two compartments are separated by an ion exchange membrane.

[0066] In some embodiments, the discharge cathode half cell reaction includes reduction of a halogen oxyanion, such as a bromine, chlorine, or iodine oxyanion, or a mixture thereof. In some embodiments, the halogen oxyanion is bromate or perbromate. In some embodiments, the halogen oxyanion is hypobromite, e.g., in acid, or bromine water (BrO ), e.g., in base. In some embodiments, e.g., embodiments in which a halogen oxyanion is used, a complexing agent is included in the anolyte and catholyte. In some embodiments, the complexing agent complexes the halogen, e.g., bromine, chlorine, iodine, or a mixture thereof, and forms a second phase with a density greater than either the anolyte or catholyte, e.g., to prevent or minimize halogen gas from forming and escaping from the liquid composition.

[0067] In some embodiments, the discharge anode half cell reaction includes oxidation of phosphite or hypophosphite, e.g., in base. In some embodiments, the discharge anode half cell reaction includes oxidation of phosphorous acid or hypophosphorous acid, e.g., in acid. In some embodiments, the discharge anode half cell reaction includes oxidation of a sulfur, tin, or chromium containing oxyanion, e.g., in acid or base.

[0068] In some embodiments, a secondary battery is provided that contains two compartments, with a first compartment containing an anolyte plus anode and a second compartment containing a catholyte plus cathode, with the first and second compartments divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte and permits an ionically conducting path between the anode and cathode. The battery includes an overall cell reaction that includes an anode half cell reaction and a cathode half cell reaction, and the anode half cell reaction includes phosphate/phosphite, phosphate/hypophosphite, phosphoric acid/phosphorous acid, phosphoric

acid/hypophosphorous acid, sulfur oxyanion/sulfur oxyanion (e.g., thiosulfate/sulfate), or formate/carbonate (e.g., in basic solution). In some embodiments, the anolyte and catholyte contain substantially the same composition prior to charging. In some embodiments, the discharge anolyte and catholyte are mixed after one or a number of charge/discharge cycle(s) to obtain substantially the same composition in the anolyte and catholyte prior to charging.

[0069] In some embodiments, a secondary battery is provided that contains two compartments, with a first compartment containing an anolyte plus anode and a second compartment containing a catholyte plus cathode, with the first and second compartments divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte and permits an ionically conducting path between the anode and cathode. The battery includes an overall cell reaction that includes an anode half cell reaction and a cathode half cell reaction. The cathode half cell reaction includes halogen oxyanion/halide and the anode half cell reaction includes phosphate/phosphite, phosphate/hypophosphite, phosphoric acid/phosphorous acid, phosphoric acid/hypophosphorous acid, sulfur oxyanion/sulfur oxyanion (e.g., thiosulfate/sulfate), formate/carbonate (e.g., in basic solution), or a metal ion (e.g., an "uncomplexed" metal cation (a single metal atom with more than one oxidation state), for example, tin. In some embodiments, the halogen oxyanion is bromate or perbromate. In some embodiments, the halogen oxyanion is hypobromite, e.g., in acid, or bromine water (BrO ), e.g., in base. In some embodiments, a complexing agent is included in the composition that contains electroactive species, i.e., the anolyte and catholyte. In some embodiments in which the halogen oxyanion includes bromine, the complexing agent complexes bromine and forms a second phase with a density greater than either the anolyte or catholyte, e.g., to prevent bromine gas from forming and escaping from the liquid composition. In some embodiments, the anolyte and catholyte contain substantially the same composition prior to charging. In some embodiments, the anolyte and catholyte are mixed after one or a number of charge/discharge cycle(s) to obtain substantially the same composition in the anolyte and catholyte prior to charging.

[0070] In some embodiments of the battery systems described herein, the secondary battery is a flow battery. Figure 1 shows an embodiment of a flow battery, with plumbing that allows mixing of the anolyte and catholyte. Discharge catholyte tank 1 and discharge anolyte tank 2 each have their own pumps 3 and 4. To charge or discharge the cell 5, valves 8 and 9 are open and valves 6 and 7 are closed. Each liquid composition, i.e., anolyte or catholyte, is only pumped to its half of the cell (or a plurality of half cells within a master cell) and then drains back to the same tank. The dashed line in cell 5 is either a separator or an ion exchange membrane which prevents mixing of the anolyte and catholyte. To mix the anolyte and catholyte, valves 8 and 9 are closed and valves 6 and 7 are open. No fluids go to the cell, the anolyte and catholyte are simply mixed until they are identical in

composition.

Examples of Secondary Battery Systems in Accordance with Considerations I, II, and III

[0071] An example of a secondary battery system in accordance with considerations

I, II, and III above is as follows. Tin may be used as the discharge anolyte active species with bromine as the discharge catholyte active species.. Tin is soluble as a halogen compound and exists in the +2 and +4 oxidation states. The two half cell reactions are: Br₃ ^" + 2e^" = 3Br^" 1.08 volts

Sn²⁺ = Sn⁴⁺ + 2e^" -.15 volts

The overall cell reaction is:

Br₃ ^" + Sn²⁺ = 3Br^" + Sn⁴⁺ 0.93 volts

[0072] An example of the proposed redox flow battery is shown in Figure 9 in the fully discharged state as it just begins to be charged. In the fully discharged state, the anolyte and catholyte are virtually the same composition and can be made identical by mixing the two preceding a charge cycle. They contain predominantly Br^" and Sn⁴⁺ ions and an adequate amount of acid to maintain the desired tin solubility in either oxidation state. The Pourbaix diagram for tin defines the desirable pH range to maintain adequate solubility. The anion from the acid should not be an additional electrochemically active species so the use of hydrobromic acid (HBr) is ideal.

[0073] When charging the battery, in the charge anolyte, Br^" is oxidized to Br₃ ^". The

Sn⁴⁺cannot be further oxidized so it is electrochemically inactive and remains unchanged. Water will not be oxidized because the bromide potential (-1.08 volts) is more favorable than the oxygen gas evolution reaction (-1.23 volts), especially when considering the overpotential associated with oxygen gas evolution. Overcharging this side of the battery will result in oxygen gas evolution and water consumption once all of the Br^" is consumed.

[0074] In the charge catholyte, Sn⁴⁺ is reduced to Sn²⁺. The Br^" cannot be further reduced so it is electrochemically inactive and remains unchanged. Hydrogen ions will not be reduced to hydrogen gas because the tin reaction at 0.15 volts is more favorable than the proton reduction reaction (0.0 volts). Once all of the Sn⁴⁺ is consumed, it is possible to reduce Sn²⁺ to the solid phase (-.14 volts). Hydrogen gas evolution will be

thermodynamically preferred, but may not occur due to the overpotential associated with hydrogen gas evolution. Care should be taken to not overcharge this compartment of the battery.

[0075] In this system, the desired reaction species overwhelmingly consume the energy during the charging process. Some extra energy may be consumed to charge the system because some of the species may diffuse in both directions across the membrane, resulting in some amount of self-discharge. Sn²⁺ will chemically react with Br₃ ^" and form Sn⁴⁺ and Br^" in the charge anolyte. Extra energy will be required to oxidize the Br^" back to Br₃ ^". Br₃ ^" will react with Sn²⁺ and form Br^" and Sn⁴⁺ in the charge catholyte. Extra energy will be required to reduce the Sn⁴⁺ back to Sn²⁺.

[0076] Once the system is fully charged, it can be discharged and is shown in Figure

10 at the start of the discharge process. The most favorable reaction is reduction of Br₃ ^" to Br^" in the discharge catholyte. Oxidation of Sn²⁺ to Sn⁴⁺ is the most favorable reaction in the discharge anolyte. Some diffusion may occur, resulting in some self-discharge. Some Sn²⁺ may diffuse into the discharge catholyte while some Br₃ ^" may diffuse into the discharge anolyte, temporarily reducing the capacity of the system. A small imbalance in species will be present after a discharge cycle. This can be fixed by mixing the anolyte and catholyte after a full discharge, for example using hardware as shown in Figure 1. To save energy and time, this mixing may be executed only periodically. For example, the system may be run for 10 cycles and then a mixing of the anolyte and catholyte is executed before the subsequent charge cycle. This will ensure that the system is returned back to its full capacity. Extra energy during charging is only needed to compensate for the species that result in self-discharge.

[0077] A factor that will greatly affect the energy density of this system is the choice of the ion exchange membrane. If a cation exchange membrane is employed and protons are the species that predominantly move during migration, the pH of the charge anolyte will increase while the pH of the charge catholyte will decrease when charging. Therefore, there must be an adequate concentration of protons in the charge anolyte to last for the entire charge cycle and still have some protons remaining. The charge catholyte does not require any more protons than necessary to maintain the Sn²⁺ and Sn⁴⁺ solubilities.

[0078] Figures 9 and 10 show the direction of cation migration across the cation exchange membrane and the way in which pH responds to charging and discharging. There is a maximum concentration of protons that is typically a function of the chemical stability of the cation exchange membrane. The starting proton concentration should therefore be approximately one half of this maximum value, i.e., enough to provide for the charge cycle depletion in the charge anolyte, the discharge cycle depletion in the discharge anolyte, and not so many in the discharge catholyte at the end of the discharge cycle or in the charge catholyte at the end of the charge cycle that the maximum proton concentration will be exceeded. The use of a cation exchange membrane results in the maximum proton concentration limiting the energy density of the system. Due to the allowable pH range, only a fraction of the potential tin concentration can be utilized.

[0079] If an anion exchange membrane is employed, the situation is different. Since both half cell reactions are pH insensitive, the migration of anions and the lack of hydroxyl ions (since both the anolyte and the catholyte are acidic) means that the pH stays relatively constant in both the anolyte and the catholyte during charging and discharging. This allows the choice of starting pH to be determined by the tin ion solubility requirements alone. In a typical example, the proton concentration can be 0.1M (pH=l), but the proton concentration can cover a wide range and still maintain adequate solubility of the tin species. One benefit of a pH of 1 is that the chemical stability of the membrane may be improved compared to a system with a much lower pH value. The other benefit is that the energy density can be determined by the solubility of one of the active species. In this example, the theoretical energy density is two times larger assuming a maximum proton concentration of 3M for the cation membrane system and a 3 M tin concentration in the anion membrane system. A larger maximum proton value will decrease the difference between the two membrane systems. The membrane would have to be stable at the unlikely proton concentration value of 6M for the energy densities to be equal. [0080] Another option is to use a separator instead of an ion exchange membrane.

However, this will allow both cation and anion diffusion and may result in a larger self discharge rate than a cation or anion exchange membrane. The pH may vary during charging and discharging because protons will move across this barrier with ease.

[0081] In a typical configuration using an anion exchange membrane, the liquid composition after it is mixed after a full discharge or when the battery is first placed into use could be as follows:

Na⁺ Sn²⁺ Sn⁴⁺ Br^" Br₃ ^" H⁺

Mixed Composition

1.9M 0M 2M 6M 0M 0.1M

[0082] After the cell is fully charged, reducing 1.9 moles of Sn⁴⁺ to Sn²⁺, the two liquids would have the following compositions:

Na⁺ Sn²⁺ Sn⁴⁺ Br^" Br₃ ^" H⁺

Discharge Catholyte

1.9M 0M 2M 0.3M 1.9M 0.1M

Discharge Anolyte

1.9M 1.9M 0.1M 6M 0M 0.1M

[0083] These are approximate numbers and do not take into account the potential diffusion of various species across the membrane during the charge cycle. Now the discharge process is executed consuming 1.9 liters per mole of Sn²⁺. At the end of the discharge cycle, the two liquids would have the following compositions:

Na⁺ Sn²⁺ Sn⁴⁺ Br^" Br₃ ^" H⁺

Discharge Catholyte

1.9M 0M 2M 6M 0M 0.1M

Discharge Anolyte

1.9M 0M 2M 6M 0M 0.1M

[0084] The two liquids are once again very similar in composition. They may be mixed again to re-establish the full capacity of the system and equalize any components that became unbalanced from diffusion across the membrane.

[0085] To make this composition, in one embodiment, tin (IV) bromide salt (SnBr₄), sodium bromide (NaBr), hydrobromic acid (HBr), and water are mixed in the desired quantities. In another embodiment, tin (II) bromide salt (SnBr₂) may be substituted for SnBr₄. To do this, tin (II) bromide salt may be substituted and electrochemically converted to the +4 oxidation state in the cell shown in figure 11. Sn²⁺ ions are oxidized at the anode (depicted on the left in Figure 11) while hydrogen gas is generated at the cathode (depicted on the right). The HBr concentration should be chosen to result in the desired proton concentration. Other tin salts can be used f so long as the anions of these salts are electrochemically inactive in the cell.

[0086] For this system, it may be beneficial to use both bromide and chloride salts.

One benefit may be improved tin solubility, which would allow a larger theoretical energy density. Another benefit may be creation of additional, electrochemically active species Br₂Cl^~ and BrCl₂ ^" with the amount and ratio depending mostly on the bromide to chloride ratio. Both of these additional anions may provide a slightly improved potential compared to the Br₃ ^~ anion, improving the energy density slightly.

[0087] The descriptions above employ stoichiometric ratios of the two active species in the mixed anolyte and catholyte after a full discharge and assume that the anolyte and catholyte are divided into equal volumes before charging. For example, there would be 2 moles per liter of Sn⁴⁺ and 6 moles per liter of Br^". The tin solubility determines the energy density in this system. Alternatively, the concentration of one of the active species may be increased beyond the amount required stoichiometrically, and the energy density will be expected to remain about the same. However, there might be a reduction in concentration overpotential for the higher concentration active species, which may increase the cell potential near the end of the discharge cycle.

Examples of Specific Redox Systems

[0088] Examples of other redox systems that satisfy conditions (I), (II), and (III), and may be mixed to form a substantially uniform composition after discharge, in accordance with (IV), are described below.

[0089] Disproportionation is thermodynamically favorable with many of these half cell reactions, but the kinetics may be slow enough to allow their use in a practical system. Thermodynamics favors the chemical formation of hydrogen or oxygen gas with some of these reactions as well, but kinetics may prevent this from occurring. These same reactions can also form hydrogen or oxygen gas electrochemically. This can be minimized by the proper choice of electrode materials, employing electrodes with high overpotentials for either hydrogen or oxygen gas evolution.

Acidic discharge cathode half cell reactions

[0090] For acidic discharge cathode reactions, a half cell potential of less than 1.23 volts will make oxygen gas evolution thermodynamically unfavorable when charging this side of the battery. Voltages exceeding this value may be acceptable when the overpotential associated with oxygen evolution is taken into account. Possible acidic discharge cathode half cell reactions include, but are not limited to:

Reactant Product

Br₃ ^" Br^¬

I₃ ^~ i^¬

CI^3" er

Br₂ Br

h Γ

Cl₂ cr

BrCV Br

io₃- Γ

cio₃- cr

Br0₄ ^" Br

io₄ ^~ Γ

cio₄ ^~ cr

Mn0₄ ^" Mn²⁺

HCrCV Cr²⁺

Ce⁴⁺ Ce³⁺

Fe³⁺ Fe²⁺

HBrO^" Br

BrO^" Br

HIO^" Γ

IO Γ

H₂S0₂ ^" HS₂0₃- [0091] With respect to the manganese-containing reaction above, the oxidized species is an oxyanion (Mn(V), while the reduced form is a simple cation (Mn⁺²). Utilizing this discharge cathode half cell reaction requires that salt formed from Mn⁺² be soluble with the anion used for the discharge anode half cell reaction.

[0092] Mixed trihalide anions including bromine, iodine and/or chlorine may also be used, for example, Brl₂ ^", ClBr₂ ^", etc. These may form when more than one halogen salt is used in the liquid composition, with the potential determined by the chemical

composition of the trihalide.

Basic discharge cathode half cell reactions

[0093] For basic discharge cathode reactions, a half cell potential less than 0.41 volts will make oxygen gas evolution thermodynamically unfavorable when charging this side of the battery. Voltages exceeding this value may be acceptable when the overpotential associated with oxygen evolution is taken into account. Possible basic discharge cathode half cell reactions include, but are not limited to:

Reactant Product

BrCV Br^¬ io₃- i^¬ cio₃- er

Br0₄ ^" Br

io₄ ^~ r

cio₄ ^~ cr

BrCV Br₂

io₃- I₂

cio₃- Cl₂

Br0₄ ^" Br₂

io₄ ^~ I₂

cio₄ ^~ Cl₂

HBrO^" Br

BrO^" Br

HIO^" r

IO r [0094] Other halogen oxyanions may also serve as candidates as the oxidized species. Some halogen oxyanions, for example, BrO^", 10^" and CIO^", are predicted to disproportionate, but may do so at a slow enough rate to be viable for use in the battery system.

Acidic discharge anode half cell reactions

[0095] Possible acidic discharge anode reactions include, but are not limited to:

Reactant Product

Sn²⁺ Sn⁴⁺

Fe²⁺ Fe³⁺

Cr²⁺ Cr³⁺

H₂S0₃ HS₂0₄ ^"

S₄0₆ ^2" S0₄ ^"

BH₄ ^" B0₂ ^"

H₃P0₃ H₃P0₄

H₃P0₂ H₃P0₄

HS0₄ ^" S₄0₆ ^2"

S₄0₆ ^2" HS₂0^3"

S0₄ ^2" S₂0₆ ^2"

HS0₄ ^" S₂0₆ ^2"

H₂S0₄ HS0₄ ^"

S0₄ ^2" H₂S0₃

H₂S0₃ HS₂0₄ ^" [0096] Other sulfur oxyanions may be used in which sulfur in a lower oxidation state is oxidized to a higher oxidation state shown in the product column, e.g., S0₄ ^2" or HS0₄ ^". It is also possible to complex cations to improve their solubility. In some cases the potential of the complexed cation half cell reaction is shifted as well.

Basic discharge anode half cell reactions

[0097] Possible basic discharge anode reactions include but are not limited to:

Reactant Product BH₄ ^" B0₂ ^"

HSn0₂ ^" + 30H^" + H₂0— > Sn(OH)₆ ^2" + 2e^~

Mn0₄ ^" + e' = Mn0₄ ^2"

S₂0₃ ^2" S0₄ ^2"

HPO^2" HP0₃ ²

HP0₂ ^" P0₄ ^3"

HC0₂ ^" + 30H^"— > C0₃ ² + 2H₂

so₃ ²- s₂o₃ ²

s₂o₃ ²- HS^"

S0₄ ^2" HS^"

so₃ ²- s₄o₆ ²-

S0₄ ^2" s₄o₆ ²-

S0₄ ^2" so₃ ²- so₃ ²- s₂o₄ ²-

[0098] Other sulfur oxyanions may be used in which sulfur in a lower oxidation state is oxidized to a higher oxidation states shown in the product column. Double discharge redox systems

[0099] Examples of specific double discharge redox systems for use in a secondary battery system as described herein appear below.

(i) Acidic systems

[00100] The following are nonlimiting examples of acidic systems.

Discharge catholyte Discharge anolyte

Br^3" Br Sn²⁺ Sn⁴⁺

ci³- cr

i³- r

[00101] There are a multitude of variations with this acidic system with 2 or 3 different halogens participating with a multitude of trihalide anions. The cell voltage increases from iodine to bromine to chlorine.

Discharge catholyte Discharge anolyte Ce⁴⁺ Ce³⁺ Sn²⁺ Sn⁴⁺

Br^3" Br^"

CI^3" CI^"

I³- r

[00102] There are a multitude of variations with this acidic system with 2 or 3 different halogens participating with a multitude of trihalide anions. The cell voltage increases from iodine to bromine to chlorine.

[00103] Discharge catholyte Discharge anolyte

Br₃ ^" Br^" H₃P0₃ H₃P0₄

Cl₃ ^" CI^" H₃P0₂ H₃P0₃

I₃ ^~ T

[00104] There are a multitude of variations with this acidic system with 2 or 3 different halogens participating with a multitude of trihalide anions. The cell voltage increases from iodine to bromine to chlorine. Each chosen discharge catholyte reaction, e.g., single, double, triple, can be combined with either the H₃P0₃/H₃P0₄ single discharge anolyte reaction or with both discharge anolyte reactions.

[00105] Discharge catholyte Discharge anolyte

Br0₃- Br- H₃P0₃ H₃P0₄

C10₃- CI- H₃P0₂ H₃P0₃

I0₃- I-

[00106] There are a multitude of variations with this acidic system with 2 or 3 different halogens participating with a multitude of oxyanions. The cell voltage increases from iodine to bromine to chlorine. Each chosen discharge catholyte reaction, e.g., single, double, triple, can be combined with either the H₃P0₃/H₃P0₄ single discharge anolyte reaction or with both discharge anolyte reactions.

[00107] Discharge catholyte Discharge anolyte

Br0₄- Br- H₃P0₃ H₃P0₄

C10₄- CI- H₃P0₂ H₃P0₃

I0₄- I- [00108] There are a multitude of variations with this acidic system with 2 or 3 different halogens participating with a multitude of oxyanions. Each chosen discharge catholyte reaction, e.g., single, double, triple, can be combined with either the H₃PO₃/H₃PO₄ single discharge anolyte reaction or with both discharge anolyte reactions.

(ii) Basic systems

[00109] The following are nonlimiting examples of basic systems.

[00110] Discharge catholyte Discharge anolyte

[00111] Discharge catholyte Discharge anolyte

HCrCV Cr ,2+ H3PO3 H₃P0₄

[00112] Discharge catholyte Discharge anolyte

I0₃ ^" Γ

[00113] One, two or three halogen basic discharge catholyte reactions can be combined with one or two discharge anolyte reactions in this basic system. Any single one of the halogen reactions can also be considered a double discharge reaction since there are intermediate oxidation states between the +5 state and the -1 state.

[00114] Discharge catholyte Discharge anolyte

Br0₄ ^" Br^" HPCV P0₄

cio₄- CI^" H₂P0² H₂P0₃

io₄- Γ

[00115] One, two or three halogen basic discharge catholyte reactions can be combined with one or two basic discharge anolyte reactions.

[00116] Discharge catholyte Discharge anolyte

Br0₄ ^" Br^" BH₄ ^" B0₂ ^"

C10₄ ^" elHSn0₂ ^" Sn(OH)₆

I0₄ ^" iMn0₄ ^" Mn0₄ ^2"

S₂0₃ ^2" sor

HC0₂- C0₃ ² [00117] One, two or three halogen basic discharge catholyte reactions can be combined with any one of the basic discharge anolyte reactions.

[00118] A multitude of other potential supplemental reaction combinations can occur by combining any of the above reactions. Chemical compatibility and solubility will determine the practicality of any system. Typically, multiple species are mixed if this provides an improvement to the energy density, economics or lifetime of the system.

System with gaseous electrode

[00119] A secondary battery may be designed with one gaseous (oxygen or air) electrode and one liquid (redox) electrode. This type of system may be implemented using acidic or basic liquid compositions, typically using a cation exchange membrane for acids or an anion exchange membrane for bases. Nonlimiting examples of discharge anolyte reactions that may be coupled with an oxygen electrode incude the following:

(i) Basic reactions

Reactant Product

BH₄ ^~ B0₂ ^~

HSn0₂ ^~ + 30H^" + H₂0— > Sn(OH)₆ ^2~ + 2e

Mn04^~ + e' = Mn04

S₂0₃ ^2" S0₄ ^2~

HPO^2" HP0₃

HP0₂- P0₄ ^3~

HC0₂ ^" + 30H^"— > C0₃ ² + 2H₂0 + 2e^~

HS0₄- S₄0₆ ^2"

S₄0₆ ^2" HS₂0^3"

S0₄ ^"2 S₂0₆ ^"2

HS0₄- S₂0₆ ^"2

H₂S0₄ HS0₄-

S0₄ ^2" H₂S0₃

H₂S0₃ HS₂0₄- (ii) Acidic reactions

Reactant Product

Sn²⁺ Sn⁴⁺

Fe²⁺ Fe³⁺

Cr²⁺ Cr³⁺

H₂S0₃ HS₂0₄ ^~

S₄0₆ ^2" S0₄ ^~

BH₄ ^" B0₂ ^"

H₃P0₃ H₃P0₄

H₃P0₂ H₃P0₄

HS0₄ ^~ S₄0₆ ^2"

S₄0₆ ^2" HS₂0₃-

S0₄ ^2~ S₂0₆ ^2"

HS0₄ ^~ S₂0₆ ^2"

H₂S0₄ HS0₄ ^~

S0₄ ^2~ H₂S0₃

H₂S0₃ HS₂0₄ ^~

Addition of Supplemental Active Species

[00120] In some secondary battery systems with anolyte and catholyte, the energy density may be limited by one of the two active species, i.e., one of the components may be limited in solubility while the other is not. This is a function of the properties of soluble salts and is related to the stoichiometric ratio of the two components in the overall redox reaction. Another possible scenario is that one or both of the half cell reactions includes protons or hydroxyl ions. Generally, in a redox battery that requires a long lifetime, compatibility of a membrane separating the anolyte and catholyte with a high concentration of protons or hydroxyl ions will be the component that limits the energy density of the battery.

[00121] In some cases where the energy density is limited by an active species in one of the two half cell reactions, it may be possible to introduce an additional species or additional half cell reaction to supplement the limiting half cell reaction. [00122] In some embodiments, one or more additional active species or half cell is added to produce a battery system that satisfies conditions (I), (II), and (III) above, and optionally may also satisfy condition (IV) with mixing of the anolyte and catholyte after discharge to fully or substantially restore the original capacity to the system. Conditions (I), (II), and (III) apply to each unique system. For example, if there are two discharge catholyte and one discharge anolyte active species, these conditions apply to discharge catholyte 1 and the discharge anolyte and the conditions also apply to the discharge catholyte 2 and the discharge anolyte.

[00123] An additional practical requirement for adding a supplemental active species is that it should not react chemically with any of the other components of the anolyte and/or catholyte when charged or discharged. However, in some embodiments, there may be some spontaneous reactions that occur due to diffusion and lead to self-discharge, just like a system with only two active species. For example, if there are two active discharge catholyte species, the higher oxidation state of the species with the larger half cell potential may spontaneously oxidize the species with the smaller half cell potential while it will be reduced. This could result from diffusion of that species from the discharge anolyte into the discharge catholyte. This will result in some self-discharge, but it will not cause a permanent capacity if the two anolyte and catholyte are mixed after a full discharge cycle and charged.

[00124] One potential complication with adding a supplemental active species is that the addition may be detrimental to the solubility of one of the original active species. For example, if the limiting original active species is soluble as a chloride salt, the supplemental active species should not be added as a chloride salt. The increase in concentration of the chloride anion will shift the equilibrium concentration of the original active species to a smaller concentration (referred to as the common ion effect). Assuming that the original species provides a higher energy density than the supplemental active species, no extra energy will be gained if the original active species concentration is decreased while the supplemental active species concentration is added.

[00125] It is also possible that adding a supplemental active species may change the limiting active species. This may be acceptable if the goal of adding an additional active species is to increase the energy density of the system. For example, a supplemental cation active species may be added as a salt of a different anion. For example, a sulfate salt may be added to a liquid composition that contains only bromide anions as the electroactive species. The solubility of the original active species must be examined to insure that they are both soluble with the newly introduced anion from the supplemental active species. The stoichiometric concentrations may need to be reduced to insure solubility of all species. This may be acceptable if an increase in energy density is gained. In some embodiments with two active species, two or more different salts of the limiting active species may be used to maximize the concentration of the active species.

[00126] The discharge curve from adding a supplemental active species will be modified, resulting in two distinct steps in the charge and discharge curves, one for each of the half cell reaction potentials. However, if the cell reactions are close in potential, steps will not be easily observed.

[00127] An example of a reaction that may benefit by addition of a supplemental species to the discharge anolyte is as follows:

Ce⁴⁺ + e^"— > Ce³⁺ 1.44 volts

Sn²⁺— > Sn⁴⁺ +2e^~ -0.15 volts

The overall cell reaction is:

2Ce⁴⁺ + Sn²⁺— > 2Ce³⁺ + Sn⁴⁺ 1.29 volts

[00128] This reaction is limited by the cerium solubility when using an anion membrane, and is further complicated by the requirement that twice as much cerium as tin is required. At a 2M cerium concentration and a 1M tin concentration, the energy density is low and only a fraction of the potential tin capacity is used. Both metals are most soluble using chlorine or bromine as the salt anion. Thus, cerium and tin solubility may be improved by adding chlorine and/or bromine anions.

[00129] Addition of bromine provides another overall cell reaction that satisfies conditions (II) and (III) above for a secondary battery as described herein:

Br₃ ^~ + 2e-— > 3Br^" 1.08 volts

Sn²⁺— > Sn⁴⁺ +2e^" -0.15 volts

The overall cell reaction is:

Br₃ ^" + Sn²⁺— > 3Br + Sn⁴⁺ 0.93 volts

[00130] This reaction is limited by the tin solubility. Although not shown for simplicity, there are potentially other possible reactions if both chlorine and bromine are employed. Br₂Cl^~ and BrCl₂ ^" may also form yielding slightly improved half cell potentials. [00131] The energy of the cell is improved by introducing a supplemental discharge catholyte active species (Br ) and the cell reaction with three active species exceeds the energy density of either of the simpler two active species cell reactions. This is due to the use of two anions to improve the maximum solubilities of cerium and tin. This system may employ an anion membrane to gain these benefits. If a cation membrane is used, the energy density may be limited by the maximum proton concentration so additional active species would not be of any benefit. Another advantage of this system using a supplemental active species is that the total bromide and/or chloride concentration per unit of energy can be reduced. This will help minimize bromine and/or chlorine vapor pressure, which will reduce this potential permanent capacity loss mechanism and reduce the environmental impact of releasing halogen to the atmosphere as a vapor.

[00132] Another example of a reaction that may benefit by addition of a

supplemental species to the discharge anolyte is as follows:

Sn²⁺— > Sn⁴⁺ +2e^~ -0.15 volts

Br₃ ^~ + 2e^~— > 3Br^" 1.08 volts

The overall cell reaction is:

Br₃ ^" + Sn²⁺— > 3Br + Sn⁴⁺ 0.93 volts

[00133] This reaction is limited by the tin solubility. By introducing tin using an alternate anion, it may be possible to increase the tin concentration. Using salts of bromine and/or chlorine will achieve this goal. In addition to the discharge cathode reaction shown above involving bromine, there are three other potential reactions that may occur when both bromine and chlorine salts are used, depending on the ratio of the anions in the liquid composition:

Cl₃ ^~ + 2e^~— > 3C1^" 1.42 volts

BrCl₂ ^~ + 2e^"— > Br^" + 2C1^" 1.2 volts (approximate)

Br₂Cl^" + 2e^~— > 2Br^" + CI^" 1.3 volts (approximate)

[00134] This system may employ an anion membrane to gain these benefits. If a cation membrane is used, the energy density may be limited by the maximum proton concentration so additional active species would not be of any benefit. A disadvantage associated with this example is the potential for both bromine and chlorine vapor to form. However, a complexing agent may be added to reduce or prevent release of halogen vapor, as described herein. [00135] An example of a supplemental reaction with only two active species is as follows:

H3PO3 + H₂0— > H3PO4 + 2H+ + 2e^~ 0.28 volts

Br₃ ^" + 2e^~— > 3Br^" 1.08 volts

The overall cell reaction is:

H3PO3 + Br₃ ^" +H₂0— > H3PO4 + 3Br^" + 2H⁺ 1.36 volts

[00136] Both phosphorus acid and phosphoric acid are weak acids. This means that only a small fraction of protons are liberated in solution. Since the minimum pH occurs after full discharge just before charging, another acid may be added to introduce an adequate quantity of protons, e.g., a strong acid with an electrochemically inert anion that does not react with any of the species in solution (for example, sulfuric acid). In this example, the energy density is determined by this maximum proton concentration (assumed to be 3M for this example). After full discharge and mixing, the proton concentration may be 3.1M, the H₃PO₄ concentration may be 1.7M and the Br^" concentration may be 4.5M. The H₃PO₄ concentration is indicated slightly greater than the stoichiometric value due to its weak acid properties. This combination will allow there to be a reasonable quantity of protons left after a full charge cycle. In this scenario, it may not be advantageous to add another active species to this system. However, an advantage may be achieved by adding another discharge oxidation reaction, the reduction in quantity of one of the species.

Another half cell reaction may occur:

H3PO2 + H₂0— > H3PO3 + 2H⁺ + 2e^~ 0.5 volts

[00137] This reaction consumes protons at the same rate as the discharge anolyte reaction with the more oxidized components shown above. Therefore, the H₃PO₄ concentration may be reduced by half in the starting solution while achieving a similar energy density but reducing the cost of this component by half. The energy density may actually improve due to the superior half cell potential of this supplemental reaction.

[00138] Another example of a supplemental reaction with only two active species is as follows:

HPO3^2" + 30H^"— > PO4^3" + 2H₂0 + 2e^~ 1.05 volts

I0₃ ^" + 3H₂0 + 6e^~— > T + 60H^" 0.26 volts

The overall cell reaction is:

3HPO₃ ^2" + I0₃ ^" +30H^"— > 3P0₄ ^3" + T + 3H₂0 1.31 volts [00139] In this example, the energy density is determined by the maximum hydroxyl ion concentration (assumed to be 3. IM for this example). After full discharge and mixing, the hydroxyl ion concentration may be 0.1M, the P0₄ ^3~ concentration may be 3M and the T concentration may be IM. After full charging, the pH will remain constant on the discharge catholyte side while the hydroxyl ion concentration will rise to 3.1 M on the discharge anolyte side. In this scenario, it may not be advantageous to add another active species to this system. However, an advantage may be achieved by adding an additional discharge oxidation reaction, the reduction in quantity of one of the species. Another half cell reaction may occur:

H₂P0₂ ^" + 30H^"— > HPO3^2" + 2H₂0 + 2e^~ 1.57 volts

[00140] This reaction consumes hydroxyl ions at the same rate as the discharge anolyte reaction with the more oxidized components shown above. Therefore, the P0₄ ^3" concentration may be reduced by half in the starting solution while achieving a similar energy density but reducing the cost of this component by half. The energy density may actually improve due to the superior half cell potential of this supplemental reaction.

Compositions

[00141] Compositions that contain electroactive species are provided for use in secondary battery systems as described herein. Typically, a composition as described herein contains electroactive species in a liquid solution. In some embodiments, the composition is aqueous. The composition may be acidic or basic. In some embodiments, a composition is provided that includes a discharge catholyte active species containing an halogen oxyanion and an anolyte active species containing phosphite, hypophosphite, a sulfur oxyanion, formate, or a metal ion {e.g., tin). In some embodiments, the halogen oxyanion is bromate or perbromate. In some embodiments, the halogen oxyanion is hypobromite, e.g., in acid, or bromine water (BrO ), e.g., in base. In some embodiments, a complexing agent is included in the composition. In some embodiments, the complexing agent complexes a halogen, e.g. , bromine, chlorine, iodine, or a mixture thereof, and forms a second phase with a density greater than either the anolyte or catholyte, e.g., to prevent halogen gas from forming and escaping from the anolyte or catholyte. In some

embodiments, the composition is chosen to satisfy conditions (I), (II), and (III) during charging and discharging of a secondary battery as described above. Design considerations for a secondary battery

[00142] The design and engineering of a secondary battery, such as a redox flow battery, is described herein. A secondary battery may be charged and discharged many times, desirably with a very small amount of capacity loss per cycle. In some embodiments, secondary battery systems described herein contain aqueous redox systems containing electroactive species. In some embodiments, the secondary battery system described herein is a flow battery.

[00143] A method for designing a secondary battery, such as a flow battery, is provided. The method includes designing a secondary battery that contains two

compartments, with one compartment containing catholyte and cathode and the other compartment containing an anolyte and anode. The two compartments are divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte, while permitting an ionically conducting path between the anode and cathode. The battery contains an overall cell reaction that contains an anode half-cell reaction and a cathode half- cell reaction. Prior to charging, the anolyte and the catholyte are substantially the same composition that predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction. The method includes selecting anode and cathode half cell reactions such that: (i) upon charging the secondary battery, in the charge anolyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and in the charge catholyte, the product of the discharge anode reaction is reduced to a lower oxidation state, and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged; and (ii) upon discharging the secondary battery, in the discharge catholyte, the product of the discharge cathode reaction is in its lowest oxidation state or the lowest oxidation state achieved during the charging and discharging of the cell, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and in the discharge anolyte, the product of the discharge anode reaction is in its highest oxidation state or the highest oxidation state achieved during the charging and discharging of the cell and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged. The method further includes selecting anode and cathode half cell reactions such that result in a secondary battery with desirable characteristics, such as thermodynamically stable chemical species, energy density in a desirable range, power density in a desirable range, energy efficiency in a desirable range, and/or reaction kinetics at a desirable rate. Considerations for designing a secondary battery with the above characteristics are described in detail below.

Chemical Stability

[00144] For a secondary battery system to maintain its capacity over a long period of time, the electroactive chemical species should be thermodynamically stable for the desired time frame during which the battery will be operational. However, it is possible to engineer a battery with thermodynamically unstable components if the conversion of those species is slow compared to the lifetime of the battery. One way of determining thermodynamic stability is through the use of thermodynamic data, reviewing free energy of formation of all of the chemical compounds of interest to gauge their stability. A more convenient method is the use of Latimer and Frost diagrams. "Latimer" or reduction potential diagrams show the standard reduction potentials connecting various oxidation states of an element. The standard reduction potential for a reduction half-reaction involving the two species joined by the arrow is shown above the arrow. Latimer diagrams show the redox information about a series of species in a very condensed form. From these diagrams a prediction may be made about the redox behavior of a given species. "Frost" or oxidation state diagrams plot the relative free energy of a species versus oxidation state. These diagrams visually show information about the properties of the different oxidation states of a species. Frost diagrams can be constructed from Latimer diagrams. The values to be plotted on the y-axis are obtained by multiplying the number of electrons transferred during an oxidation state change by the standard reduction potential for that change. Thermodynamic stability is found at the bottom of a Frost diagram. Thus, the lower a species is positioned on the diagram, the more thermodynamically stable it is (from a oxidation-reduction perspective). A species located on a convex curve can undergo disproportionation, and those species on a concave curve do not typically disproportionate.

[00145] These diagrams quickly show if a substance will disproportionate.

Disproportionation results in the conversion of a species in one oxidation state into two species with one higher and one lower oxidation state. A thermodynamically unstable substance may cause a permanent loss of capacity to a battery system by essentially consuming the reaction products and creating species that may no longer take place in the half cell reactions. This problem is intensified if one of the products of disproportionation is a gas. Additionally, comproportionation can occur if there are two species present that react to form a third species with a negative free energy of formation.

[00146] Another scenario that can modify the behavior of a redox flow battery is the oxidation of electroactive species by oxygen in the air. This could be considered a positive attribute if this occurs in the discharge catholyte, since oxidizing the active species after discharge is fundamentally the same as charging the active species with no stored electrical energy consumed to execute this reaction, but charging would still required for the discharge anolyte. Further, oxygen could possibly oxidize the active species in the discharge anolyte before the battery is discharged which is essentially discharging that active species without getting the benefit of the energy from that reaction. Overcharging the battery to attempt to fix either of these scenarios may result in the formation of hydrogen or oxygen gas and may change the pH of the anolyte or catholyte. Oxidation is a serious consequence because it has the potential to permanently modify the capacity of the system. Having the anolyte and catholyte storage tanks sealed may help minimize this problem, but concerns over small amounts of gas and pressure from temperature changes requires a more complex solution to this issue. It may be necessary to add small amounts of acid, base and/or water to the anolyte and/or the catholytes to minimize or avoid this problem. This is an important consideration if the battery is to maintain its capacity over many years. The reaction kinetics of oxidation of the various chemical species is a determining factor with regard to the speed with which these potential reactions may occur.

[00147] Carbon dioxide is present in air at approximately 400 ppm. If permitted, this gas can react with an aqueous solution to form carbonate ions. This can cause the precipitation of certain salt species, especially in basic solutions. Minimizing the exposure of the anolyte and catholyte to air will address this potential problem.

High Energy Density

[00148] To maximize the energy density of a secondary battery, e.g., a flow battery, factors that should be considered include, but are not limited to, solubility of reactant and product species in the anolyte and catholyte, maximization of the operational voltage of the battery, and maximization of number of electrons per reaction. "Energy density" refers to energy per unit volume. Typical units are watt-hr/liter.

Reactant and product solubilities

[00149] The reactant and products species must be highly soluble in the anolyte and catholyte. In some embodiments, the anolyte and catholyte are aqueous and the reactants and products are soluble in water. There may be some interaction of the active species with other components that are present in the anolyte and/or catholyte so this creates a potentially complex analysis. For instance, the pH of the solution can have a dramatic effect on the solubility of certain species. The use of Pourbaix diagrams helps establish desirable pH values to maximize solubilities. A Pourbaix diagram is a type of phase diagram that displays the thermodynamically favorable phase as a function of the pH and electrochemical potential. If the active species requires protons or hydroxyl ions and water in the reaction, the pH may determine the energy of the system.

Maximization of cell voltage

[00150] When a voltage is applied to water, no current will pass until the potential reaches a value where water is reduced at one electrode to form hydrogen gas and oxidized at the other electrode to form oxygen gas. Thermodynamically, this reaction should start to occur when 1.23 volts is applied to the two electrodes. However, the kinetics associated with these reactions requires the voltage to be significantly higher before bubbles are generated at atmospheric pressure. The active ionic species in the anolyte and catholyte must preferentially react in the two half cell reactions at the electrodes. The two desired electrode reactions should be more favorable than hydrogen or oxygen gas evolution during the charging process. Theoretically, this limits the maximum cell voltage to 1.23 volts, but practically, larger potentials may be generated without any significant hydrogen or oxygen evolution.

[00151] In an acidic solution, the reduction half cell potential associated with hydrogen gas evolution occurs at 0.0 volts. This reaction is arbitrarily chosen to occur at 0.0 volts and all other half cell potentials are expressed in relation to this reaction. This means that the potential associated with oxygen evolution must occur at a magnitude of 1.23 volts away from this reaction. By convention, the half cell potential of oxidation of acidic water to form oxygen gas is -1.23 volts. Thus, the half cell potential of the acidic charge anolyte oxidation reaction should to be greater than -1.23 volts (to avoid oxygen evolution) while the half cell potential of the acidic charge catholyte reduction reaction should be greater than 0 volts (to avoid hydrogen gas evolution). Practically, these values can be exceeded by many hundreds of millivolts.

[00152] In a basic solution, the potential associated with the reduction of water to hydrogen gas occurs at -0.83 volts while the oxidation potential for oxygen gas evolution occurs at -0.41 volts yielding the same 1.23 volt potential difference range. The potentials at which hydrogen gas and oxygen gas are generated in a basic solution are shifted due to the change in activities of protons and hydroxyl ions (quantitatively determined using the Nernst Equation). It is much more difficult to generate hydrogen gas in a basic solution due to the extremely small proton concentration and much easier to generate oxygen gas due to the high concentration of hydroxyl ions. Ideally, the half cell potential of the basic charge anolyte oxidation reaction should be greater than -0.41 volts (to avoid oxygen evolution) while the half cell potential of the basic charge catholyte reduction reaction should be greater than -0.83 volts (to avoid hydrogen gas evolution) during the charge process.

[00153] Some Pourbaix diagrams display practical phase boundaries that take into account the overpotentials associated with hydrogen and oxygen gas evolution. These lines are parallel to the thermodynamically generated water stability lines. They typically show a stability range of approximately 2 volts over the entire pH range from strong acid to strong base compared to the 1.23 volt theoretical thermodynamic range. When discussing potential half cell reactions for secondary batteries in this document, this practical stability range is assumed, but it is important to be aware of possible gaseous reactions in such instances. As an example, an aqueous lead-acid battery has an open circuit voltage exceeding 2 volts and has minimal hydrogen gas evolution. The acceptable capacity loss over the lifetime of the battery and the charging efficiency will determine how much gas evolution is acceptable.

[00154] The stability range of aqueous compositions varies linearly with pH between the limits discussed above. There is a complex relationship between the active ionic species and the pH that will determine the likelihood of hydrogen or oxygen gas evolution.

Additionally, the magnitudes of the overpotentials associated with the evolution of hydrogen and oxygen vary with pH as well. While some reactions are quite simple and only involve a single cation with two oxidation states (e.g. , chromium or iron), other reactions involve protons or hydroxyl groups and require the pH to be within a specific range. Also, the solubility of some species is strongly dependent on the pH. Many of desirable cation species for redox reactions have high solubility in acidic solution and essentially no solubility in basic electrolytes.

[00155] Another consideration is the resistance. If the concentration of ionic species is low (like neutral water), it will have a large resistance which will convert energy to heat when current is passing through the cell. Neutral salts may be added to reduce the resistance, but those ionic species may interfere with the cell reactions or decrease the solubility of an active species. Sometimes adding acids or bases is a better method to decrease the resistance of the liquid composition.

[00156] Another tool that can be used to extend the useful voltage range of the liquid composition containing electroactive species involves the choice of electrode materials. Although these electrodes are inert and do not take place in the half cell reactions, the overpotential associated with hydrogen and oxygen gas evolution is a strong function of the electrode material. For secondary batteries, e.g., flow batteries, it is desirable to minimize the formation of either gas, so materials with high overpotentials may be desirable.

[00157] An electrochemical reaction is not required to form hydrogen or oxygen gas in a desired liquid composition. Rather, a direct chemical redox reaction between available species may occur. This is a disadvantage of a half cell potential outside the stability range of the electroactive species, balanced against the advantage of a larger cell potential generating more energy per mole of active species.

[00158] For example, when chromium ions are present in an acidic solution, thermodynamic predictions indicate that hydrogen gas should form:

Cr⁺²— > Cr⁺³ + e^" 0.41 volts

2H⁺ + 2e^~— > H₂(gas) 0.0 volts

The overall chemical reaction is:

2Cr⁺² + 2H⁺— > 2Cr⁺³ + H₂(gas) 0.41 volts

[00159] A positive potential means that it has a negative free energy of formation and the reaction is spontaneous. Many factors are involved in determining if hydrogen gas will actually be generated and how much will be evolved. One factor that can help minimize gas evolution in this example is not to have any metal contact the liquid composition. Much like a corrosion process or an electroless deposition process, a conductive surface provides a low impedence path for electrons which may aid in the completion of the reaction. This becomes a challenge when it comes to the electrode which must be an electronic conductor. However, the overpotential associated with hydrogen or oxygen gas reactions is a strong function of the material and high overpotential materials are available. Ideally, gas evolution should be avoided in this type of cell but it is possible for a chemical and an electrochemical gas evolution process to occur at an electrode. This will modify the liquid composition in some manner and have some impact on the capacity of the system.

[00160] Another example involves borohydride in a basic solution:

BH₄ ^~ + 80H^"— > B0₂ ^~ + 6H20 + 8e^~ 1.24 volts

2H₂0 + 2e— > H₂(gas) + 20H^" -0.83 volts

The overall chemical reaction is:

BH₄ ^" + 2H₂0— > B0₂ ^" + 4H₂ (gas) 0.41 volts

This reaction with a positive voltage and a negative free energy of formation can be used to generate hydrogen gas. It has practical applications since the concentration of stored hydrogen in borohydride is quite large and provides a safe method of storing and transporting hydrogen.

[00161] An example of an acidic solution that may evolve oxygen gas involves cerium in acidic solution:

Ce⁺⁴ + e^"— > Ce⁺³ 1.44 volts

2H₂0— > 0₂(gas) + 4H⁺ + 4e^~ - 1.23 volts

The overall chemical reaction is:

4Ce⁺⁴ +2H₂0— > 4Ce⁺³ + 0₂(gas) + 4H⁺ 0.21 volts

[00162] One possible method to maximize the cell potential is to use an acidic solution for the discharge cathode and a basic solution for the discharge anode. This would yield a thermodynamic potential range of 2.06 volts before hydrogen or oxygen gas is evolved (even higher when overpotentials are considered). Number of electrons per reaction

[00163] The number of electrons associated with a half cell reaction is directly related to the generated energy. For example, when one iron cation is reduced and one chromium ion is oxidized during the discharge of an iron chromium redox battery, one electron flows in the external circuit. The energy of this electron is related to the cell voltage. Some half cell reactions involve more (or less) than one electron. For example, with zinc reduction, it takes two electrons to reduce the ion to a metal. In this case, two electrons flow in the external circuit. This provides twice the amount of energy in comparison with a single electron process (assuming the same cell potential). All things being equal, the more electrons associated with the cell reactions, the higher the energy and energy density. Looking at it another way, of the active species may be used at half the concentration in a half cell reaction involving two electrons in comparison with a half cell reaction at the same potential involving a single electron, at the same energy density.

Reaction Kinetics

[00164] Analyzing the thermodynamic properties associated with a proposed battery system predict whether the cell reaction is spontaneous, at what potential it will be executed close to equilibrium, and how much material can be dissolved into the liquid composition that contains electroactive species. These parameters establish the theoretical parameters of a given battery system. However, the reaction kinetics associated with the system must be appropriate for the desired energy and power requirements. Typically, the simpler the reaction, the faster it will go. For example, the reduction of a +3 iron cation only requires the presence of the ion and an electron for the reaction to proceed to a +2 iron cation. Some reactions require multiple ions to all be present simultaneously for a reaction to proceed, which may result in the reaction proceeding at a slower rate than a simpler reaction.

However, in some instances, a simple reaction may also proceed relatively slowly, for example, the simple chromium +3 reduction to chromium +2. The macroscopic

consequences of slow reaction kinetics at the atomic level are typically a relatively larger loss of voltage compared to the open circuit potential of the reaction at a given current density compared to a half cell reaction with fast kinetics. However, the use of kinetically slow species may still be advantageous in some circumstances in view of economic considerations. More cells or higher surface area electrodes may be employed to compensate for the lower potential power. Multiple Phase Systems

[00165] In some embodiments, a secondary battery is provided in which an additional phase is created or consumed during the operation of the cell. In some redox battery systems in which halogens are used, a halogen species is formed with an oxidation state of 0. The halogen species may be soluble in the anolyte and catholyte but with a significant vapor pressure such that it may escape from the liquid as a halogen gas over time. Any single or multiple combination of chlorine, bromine or iodine in the 0 oxidation state may form a gaseous phase with a significant vapor pressure. Such a halogen gaseous phase may serve as a health hazard, a source of corrosion, and/or a capacity loss mechanism for the battery. In some embodiments, one or more complexing agent(s) may be included to minimize or eliminate production of halogen gas. (See, e.g., U.S. Patent Nos. 4,038,459 and 4,038,460.

[00166] A complexing agent may desirably include one or more of the following properties:

-low cost

-adequate aqueous solubility

-complexes multiple halogen atoms

-electrochemically inactive in the aqueous state

-complex forms a low viscosity liquid (not solid or high viscosity liquid)

-density of complexed phase more dense than anolyte or catholyte (for example, density greater than about 1.2 g/cm3)

-complexed phase has a low freezing point (liquid at battery operating temperature, for example, about 10°C or higher)

-high separation coefficient such that the aqueous halogen 0 oxidation state concentration is low (low aqueous vapor pressure) and complex phase concentration is high (allows relatively high energy density)

[00167] In one embodiment, the discharge catholyte includes bromine as an active species, and the anolyte and catholyte contain a complexing agent. The reversible half cell reaction is as follows:

Br₂ + 2e^~— > 2Br^~ 1.08 volts

[00168] When the battery is fully charged, most of the bromine is complexed in the 0 oxidation state in an oil-like phase. This material has a density greater than the aqueous anolyte and catholyte in a secondary battery, e.g., it will sink in the tanks holding the liquids in a redox flow battery. To discharge the battery, this oil phase must somehow make contact with the inert cathode so that it can be reduced to bromide ions. One way to do this is to emulsify the oil and aqueous solution and deliver it to the cathode. The electrically conducting cathode, the oil and the aqueous phase are all simultaneously present allowing the spontaneous reduction reaction to proceed. Pumping this emulsified mixture past the cathode allows the reaction to proceed without concentration polarization until the oil phase is depleted. As the complexed bromine is consumed from the oil, the complexing agent reverts to a soluble aqueous species reducing the quantity of oil. To charge the system, the aqueous phase is pumped to the anode and bromide is oxidized to the 0 oxidation state (bromine). It then chemically reacts with the complexing agent in the aqueous phase and forms the oil. When this oil is returned to the tank, it eventually settles due to its higher density. One may deliver an emulsified solution to the anode once the oil phase starts to form, but it is not necessary for an efficient charging process. In fact, oil in contact with the anode during charging may decrease the available electrode surface area and increase the overpotential, wasting energy and degrading the voltage efficiency. Using bromine and a complexing agent in this manner yields 1 electron for each bromine atom. Preferably, the solubilities of all the required species are high and the separation coefficient of the complexing agent may be large, allowing a system with an advantageously large energy density.

[00169] Nonlimiting examples of complexing agents that may be used in halogen- containing secondary battery systems described herein include N-ethyl N- methylmorpholinium bromide (abbreviated as "MEM" or "EMMB"), N-ethyl N- methylpyrrolidinium bromide (abbreviated as "MEP" or "MEPB"), tetra-butyl ammonium bromide (abbreviated as "TBA" or "TBB"), dimethylethylpropyl ammonium bromide

(abbreviated as "2EMP"),and dimethylethyl ammonium bromide (abbreviated as "2M2E"). (See, for example, Cathro et al. (1986) Journal of Power Sources 18, 349-370.)

[00170] An alternative system than includes bromine and a complexing agent is provided herein that may improve the energy density, requires less complexing agent, and requires less bromine species. This system uses a bromate ion as the reactant and bromide as the reduced product of the discharge cathode reaction. An advantage of this system is that 6 electrons are extracted from each bromine atom compared to 1 electron in a system using bromine. A potential disadvantage of using bromate ions is that other cation species present in the liquid composition containing electroactive species must form a soluble bromate salt to avoid precipitation. Typically, bromide salts are more soluble than bromate salts, but six times less bromate is required for the same energy density in the secondary battery system described herein. In one embodiment, another anion species is included as the discharge anolyte electrochemically active species. Typically, sodium or ammonium cations can then be used as a source of these other active species and avoid any

precipitation problems since these are adequately soluble as bromates. There may also be kinetic disadvantages associated with the bromate half cell reactions compared to the bromine/bromide reaction.

[00171] An example of an acidic system including a bromate in a flow battery is described below. In this example, the discharge anolyte electrochemically active species is soluble and does not interfere with the bromine-containing reactions. "A" is an ionic species that can participate in a reversible electrochemical reaction.

Br0₃ ^" + 6H⁺ + 6e^"— > Br^" + 3H₂0 1.44 volts

A³⁺— > A⁵⁺ + 2e^" 0 volts

The overall cell reaction is:

Br0₃ ^" + 3 A³⁺ + 6H⁺— > Br^" + 3 A⁵⁺ + 3H₂0 1.44 volts

[00172] A second phase is present only during the intermediate steps of the reaction and not at the full discharge or full charge state. In some embodiments, after discharge, the anolyte and catholyte are mixed or not mixed and are replaced in the discharge catholyte and discharge anolyte tanks. Any oil that is present in the discharge anolyte tank may be moved back to the discharge catholyte tank.

[00173] An example of a redox flow battery incorporating this system is shown in Figure 2 in the fully discharged state. In the fully discharged state, the anolyte and the catholyte contain essentially the same composition and can be made identical by mixing the two liquids preceding a charge cycle. They contain predominantly Br^"' A⁵⁺ ions, protons and other electrochemically inactive species. There is virtually no oil phase present because there is no bromine. When the system is fully discharged, only aqueous solutions need to be mixed between the two tanks. When the system is partially discharged, some oil may be present but still, only the aqueous phase needs to be moved between tanks. After mixing a system that was not fully discharged, the bromate and complexed bromine will react with the remaining A³⁺ to form bromide and A⁵⁺. This will result in a system containing very little oil phase after mixing, similar to a fully discharged system. Since there is virtually no oil present after a discharge and mixing procedure, there is no concern about equalizing the complexed bromine containing species with this system. Any remaining oil in the discharge anolyte tank may be moved to the discharge catholyte tank.

[00174] One possible method for moving oil from the discharge anolyte tank to the discharge catholyte tank without an additional pump is shown in Figure 3. The discharge anolyte tank 1 is at a higher elevation than the discharge catholyte tank 2. The bottom of the discharge anolyte tank 1 should be higher than the top of the oil phase 3 in the discharge catholyte tank 2 when all of the oil phase is in the discharge catholyte tank, plus a small amount for the plumbing. The top of the aqueous solution 4 in the discharge anolyte tank 1 should also be higher than the top of the aqueous phase 5 in the discharge catholyte tank 2 by an amount greater than or equal to the height of the oil in the discharge anolyte tank 1. Typically, the volume of aqueous solutions is the same in both tanks, but the requirement is only that there be sufficient force from gravity to drive the oil from the discharge anolyte tank 1 to the discharge catholyte tank 2 without the aid of a pump. After mixing and allowing some time for all of the oil phase to settle down to the bottom of the discharge anolyte tank 1 , a valve 6 is opened. A sensor 7 monitors the liquid and shuts the valve 6 when the solution changes from predominantly oil phase to predominantly aqueous phase. Any suitable sensor, such as an optical or capacitative sensor, may be used to distinguish between the two liquids. This may alternatively be accomplished without the use of a sensor by pumping an experimentally determined quantity of fluid. Optional fluid level sensors 8 and 9 can be used to determine the tank fluid volumes. The plumbing

arrangement shown in Figure 1 (or alternative configurations) could then be used to adjust the liquid volumes in the two tanks to the desired levels.

[00175] A similar configuration but with two different tank diameters may be employed, as shown in Figure 4. In this configuration, valve 6 is opened (after mixing and allowing some time for all of the oil phase to settle down to the bottom of discharge anolyte tank 1) until gravity equalizes the top fluid levels in both tanks. Ideally, the quantities of fluid in each tank are equalized and no oil remains in the discharge anolyte tank 1. The tank diameters and tank height differences may be chosen such that the energy content of the discharge anolyte is equal to the energy content of the discharge catholyte. Sensor 7 may be used to experimentally confirm proper operation when first setting up the system and/or may be used during operation to confirm that all of the oil is transferred to the discharge catholyte tank 2.

[00176] Figure 5 shows a configuration which allows the mixing of the anolyte and catholyte following a full or nearly full discharge, using the method described above and shown in Figure 3. To mix the anolyte and catholyte, valve 9 is closed and valves 7 and 10 are open and pump 3 is turned on. As fluid is introduced into the discharge anolyte tank 2, gravity forces fluid through the line with valve 10 back to the bottom of discharge catholyte tank 1. Pump 3 is stopped and valves 7 and 10 are closed when adequate mixing is achieved. In this scenario, pump 4 is not required for mixing and would only be turned on to pump liquid through the cell so no valves are required in that line. After mixing and waiting for any remaining oil to settle to the bottom of the discharge anolyte tank, the oil may be moved back to the discharge catholyte tank. Valves 9 and 10 are closed and valve 7 is open and pump 3 is turned on to create the desired height difference between the two tanks. Enough liquid should be pumped so that all of the oil will be returned to the discharge catholyte tank 1 before the two fluid levels are equal in the two tanks. Valve 7 and pump 3 are turned off and valve 10 is opened. The fluid stops flowing when the level of the two liquids are equal. A sensor 11 may monitor the liquid and indicate when the solution changes from predominantly oil phase to predominantly aqueous phase.

[00177] Both tanks can be at the same height (or any height variation) and a pump 1 can be used to transfer any oil from the discharge anolyte tank 2 to the discharge catholyte tank 3, as shown in Figure 6. A sensor 4 can indicate when the oil phase or the aqueous phase is pumped. If the pump does not allow fluid flow when it is off, nothing else is required. If the pump allows fluid to flow when it is off, a valve 5 can be opened when pumping starts and closed when the sensor 4 indicates that the oil phase is depleted.

Sometimes fluid can flow through a pump when it is off if the liquid is higher than the pump driven by gravity. Some pumps won't allow this even if gravity is pushing on the fluid. Mixing may be done after a full or partial discharge as described previously, for example, using the system configuration shown in Figure 1.

[00178] After optionally moving remaining oil in the discharge anolyte tank to the discharge catholyte tank, and optionally mixing the anolyte and catholyte, as described above, the system may be charged. One possible method involves pumping the aqueous charge anolyte to the charge anolyte side of the cell where the aqueous bromide is oxidized to bromine, hypobromite and/or bromate. Any bromine reacts with the aqueous complexing agent and forms the oil phase. As this solution is returned to the discharge catholyte tank, the oil phase settles to the bottom of the tank due to its higher density compared to the aqueous discharge catholyte. When most of the bromide is consumed, the potential will start to rise because there are less reactant species left in the aqueous solution. At a time preceding this voltage rise, an emulsified solution of the charge anolyte is fed to the charge anolyte side of the cell. Complexed bromine from the oil phase, hypobromite and bromide are oxidized to bromate to complete the charge process. Complexing agents are released from the oil and become aqueous components. During this charge, hypobromite may disproportionate generating bromate and bromide. By charging in two steps, the quantity of bromide present at the same time as bromate is minimized, thus minimizing the chemical reaction between bromide and bromate to form bromine. Six electrons are consumed per mole to convert all of the initial bromide to bromate in a scenario in which bromate and bromide are not present simultaneously.

[00179] Another method includes feeding an emulsified solution of the charge anolyte to the charge anolyte side of the cell. As bromine is formed and complexed, the oil phase will increase in volume and allow an emulsified solution to be formed. The potential associated with the bromide/bromine oxidation reaction is more favorable than the bromide/bromate oxidation reaction. Thus, the kinetics may be more favorable for the bromide/bromine reaction than the bromide/bromate reaction, and bromine may be more likely to form than bromate in this scenario. There is one potential advantage associated with this method. The predominant reason for using complexing agents is to minimize the vapor pressure of bromine. Escaping bromine vapor may permanently degrade the capacity of the system and create corrosion and health concerns. Always pumping an emulsified solution will minimize the amount of bromine and the maximum bromine vapor pressure during the discharging and charging of the cell, in comparison with the other method described above, in which an aqueous solution is pumped first, followed by pumping of an emulsified solution. Another potential benefit is reduction of the required concentration of the complexing agent(s), which may be a relatively expensive component of the liquid composition. [00180] When charging is initiated, bromide is oxidized to bromine, hypobromite and/or bromate. The presence of an emulsified solution allows the simultaneous oxidation of complexed bromine to hypobromite and/or bromate. This will result in a lower concentration of complexed bromine in comparison to a method in which an aqueous solution is pumped at the beginning of the charge cycle.

[00181] In both of the charging methods described above, the volume of oil is at its maximum value when the bromide concentration approaches its minimum value. Once the bromide concentration is depleted, the complexed bromine will be oxidized to bromate. At any time during the charging, any bromide that is present will react with bromate, forming bromine, which will then be complexed. This newly- formed complexed bromine will then be oxidized to bromate. Advantageously, only 6 electrons are required per mole to oxidize all of the bromide to bromate, even when the chemical reaction between bromide and bromate occurs. This is because the comproportionation reaction between bromide and bromate lowers the oxidation state of the one bromate species, but it raises the oxidation state of the five bromide species, leaving the average oxidation state of bromine species unchanged. The A⁵⁺ ions present in the charge anolyte are electrochemically inactive since they are in their highest oxidation state or their highest oxidation state achievable under the potentials experienced during the charging process.

[00182] The fully charged cell at the start of discharge is shown in Figure 7. Since the reactant species in the discharge catholyte is bromate, only the aqueous phase is required. There is virtually no oil present in the discharge catholyte tank since most of the bromine is in the form of aqueous bromate. It would be advantageous to pump just aqueous solution from the discharge catholyte tank to avoid the presence of oil at the cathode during discharge. Oil in contact with the cathode may decrease the available electrode surface area and increase the overpotential, wasting energy during discharge. One option is to have two different feed tubes from the discharge catholyte tank 1 to the pump 2 as shown in Figure 8. One tube 3 that feeds an emulsified solution may be used for the entire or second part of the charge process and a second tube 4 that feeds just aqueous solution may be used for the discharge process or the first part of the charge process. A pump 5 can be used to create an emulsion by having both oil phase 6 and aqueous solutions 7 fed to it. Ultrasonic energy can alternatively be employed to create an emulsion. Valves can be used to switch between the two configurations. When valve 8 is closed and valve 9 is open, aqueous solution from pump 2 is fed to the discharge catholyte side of the cell. When valve 8 is open and valve 9 is closed, an emulsion from pump 2 is fed to the charge anolyte side (same as the discharge catholyte side) of the cell.

[00183] After the cell is fully charged, as shown in Figure 7, the system may be discharged. One method includes pumping the aqueous catholyte to the discharge catholyte side of the cell where the aqueous bromate is reduced to hypobromite, bromine and/or bromate. Any bromine reacts with the aqueous complexing agent and forms the oil phase. As this solution is returned to the discharge catholyte tank, the oil phase settles to the bottom of the tank due to its higher density compared to the aqueous discharge catholyte. When most of the bromate is consumed, the potential will start to drop because there are less reactant species left in the aqueous solution. At a point in time preceding this voltage drop, an emulsified solution of the discharge catholyte is fed to the discharge catholyte side of the cell. Complexed bromine from the oil phase, hypobromite and bromate are reduced to bromide to complete the discharge process. Complexing agents are released from the oil and become aqueous components. During discharge, hypobromite may disproportionate generating bromate and bromide. By discharging in two steps, the quantity of bromide present at the same time as bromate may be minimized, thus avoiding chemical reaction between bromide and bromate to form bromine. Six electrons per mole are generated by converting all of the initial bromate to bromide in a scenario in which bromate and bromide are not present simultaneously.

[00184] Another method for discharging the system involves feeding an emulsified solution of the discharge catholyte to the discharge catholyte side of the cell. There is no oil present at the beginning of the discharge process, so the solution starts as an aqueous solution. Any bromine that is formed will be complexed and the oil phase will increase in volume, forming an emulsified solution. Bromate, hypobromite and/or complexed bromine may be reduced.

[00185] The volume of oil is at its maximum value when the bromate concentration approaches its minimum value in both discharge methods described above. Some reduction of complexed bromine will occur simultaneously with bromate reduction. At any time during discharge, bromide that is present will react with bromate, forming bromine, which will then be complexed. This newly formed complexed bromine will then be reduced to bromide. Only 6 electrons per mole are required to reduce all of the bromate to bromide, even with the chemical reaction occurring between bromide and bromate. This is because the comproportionation reaction between bromide and bromate lowers the oxidation state of the one bromate species, but it raises the oxidation state of the five bromide species, leaving the average oxidation state of bromine species unchanged. The A⁵⁺ ions present in the charge anolyte are electrochemically inactive since they are in their highest oxidation state or the highest oxidation state achievable under the potentials experienced during the charging process.

[00186] Similar to the charging process, always attempting to pump an emulsified solution during the discharge will minimize the amount of bromine and the maximum bromine vapor pressure, in comparison to a method in which an aqueous solution is pumped first, followed by pumping of an emulsified solution. When discharge is initiated, bromate is reduced to hypobromite, bromine or bromide. The presence of an emulsified solution allows the simultaneous reduction of complexed bromine to bromide. This will result in less complexed bromine in comparison to a method in which an aqueous solution is pumped at the beginning of the discharge cycle. As the amount of bromate decreases, a greater amount of complexed bromine will be reduced. A method in which only emulsified solution is pumped allows a lower concentration of complexing agent(s) to be used, because the maximum quantity of complexed bromine will be lower than a method in which an aqueous solution is pumped first, followed by pumping of an emulsified solution.

[00187] In the discharge anolyte, A³⁺ will be oxidized to A⁵⁺. Since there is virtually no oil phase present at the anode, bromine is essentially absent. Any trace amounts of bromine or bromide will not be oxidized due to their unfavorable oxidation potential compared to oxidation of A³⁺.

[00188] Examples of reaction paths that may occur in this system are provided below.

[00189] If the bromate is reduced to HOBr, this reaction will yield 4 electrons per mole.

Br0₃ ^~ + 5H⁺ + 4e^~— > HOBr + 2H₂0 1.51 volts

The HOBr may disproportionate into bromine and bromate:

5HOBr = 2Br₂ + Br0₃ ^" +2H₂0 + H⁺

20% of the bromine would then be in the form of bromate, which can then be reduced, yielding 1 more electron per mole (5 electrons for 20% of the bromine). All the bromine in the 0 oxidation state (complexed bromine) can be reduced to bromide, yielding one more electron per mole:

Br₂ + 2e^~— > 2Br^~ 1.09 volts

The total yield is 6 electrons per mole even with this indirect reaction path. However, this requires adequate kinetics of the HOBr disproportionation reaction to generate the bromate to continue the reaction. After a full discharge, the bromine is predominantly present as bromide. It will take 6 electrons to convert the bromide back to bromate during the charge cycle.

[00190] In another reaction path, the bromate is reduced to HOBr, yielding 4 electrons per mole were yielded:

Br0₃ ^~ + 5H⁺ + 4e^~— > HOBr + 2H₂0 1.51 volts

If the disproportionation reaction is slow and only partially completed, the following electrochemical reaction may occur along with the chemical disproportionation reaction: 2HOBr + 2H⁺ +2e^"— > Br₂ +2H₂0 1.6 volts

5HOBr— > 2Br₂ + Br0₃ ^~ +2H₂0 + H⁺

The fraction of HOBr electrochemically reduced to bromine will yield 1 electron per mole, while the remaining fraction of the bromine present as bromate can then be reduced, also yielding 1 more electron per mole. The yield is 5 electrons per mole at this point. All the bromine in the 0 oxidation state (complexed bromine) can be reduced to bromide, yielding one more electron per mole:

Br₂ + 2e^~— > 2Br^" 1.09 volts

After a full discharge, the yield is 6 electrons per mole and the bromine is predominantly present as bromide. It will take 6 electrons to convert the bromide back to bromate during the charge cycle.

[00191] If the bromate is reduced to bromide, the following half cell reaction will be executed:

Br0₃ ^" + 6H⁺ + 6e^~— > Br^" + 3H₂0

As this reaction proceeds, the bromide will react with bromate to form bromine according to this spontaneous reaction:

Br0₃ ^" + 5Br^~ +6H⁺— > 3Br₂ + 3H₂0

Since five bromide molecules are required to reduce one bromate molecule, up to 17% of the bromate can be chemically consumed and not take place in the half cell discharge reaction, yielding 5 electrons per mole (83% of 6 electrons). When all of the bromate is consumed, the complexed bromine can be reduced to bromide, yielding 1 more electron per mole. Once again the total yield is 6 electrons per mole. Since all of the bromine is present as aqueous bromide, it will take 6 electrons to convert it back to bromate during the charge cycle.

[00192] Any of the discharge reaction paths discussed above result in aqueous bromide as the final product of the discharge catholyte reaction, yielding six electrons per mole, and six electrons per mole are required to convert the bromide back to bromate.

Other reaction paths combining these reactions are possible, all yielding 6 electrons per mole when the starting reactant is bromate and the final product is bromide.

[00193] Additional reduction reactions are possible, such as reduction of perbromate to bromate, perbromate to HOBr, perbromate to bromine, or perbromate to bromide.

Comproportionation reactions may occur, such as perbromate plus HOBr or perbromate plus bromide. The yield will be the same for all perbromate reaction paths, 8 electrons per mole for bromide production.

[00194] With any of the above starting reactants and final products, the energy extracted during discharge and the energy required for charging will vary with the specific reaction path. This will be a result of the kinetics associated with each specific

electrochemical step, which modifies the associated overpotential of the reaction, as well as the specific potentials associated with each half cell reaction. Disproportionation and/or comproportionation reactions will impact the energy values as well by varying the amount of product produced, which is then consumed in an electrochemical reaction.

Examples of Discharge Catholyte Systems

[00195] Nonlimiting examples of specific discharge catholyte systems are as follows:

Bromate/bromide (acidic) 6 electrons per mole

Perbromate/bromide (acidic) 8 electrons per mole

HBrO/bromide (acidic) 2 electrons per mole

Bromate/bromide (basic) 6 electrons per mole

Perbromate/bromide (basic) 8 electrons per mole

BrOVbromide (basic) 2 electrons per mole

Iodine, chlorine, or mixed halogen analogues of the above bromine-containing systems [00196] Different intermediate products will occur, but the yield will be the same for each process independent of the reaction path.

Examples of Discharge Anolyte Systems

[00197] Non-limiting examples of specific discharge anolyte systems are as follows:

H₃PO₃/H₃PO4 (acidic)_

H₃PO₂/H₃PO4 (acidic)

S₂0₆ ²7S0₄ ^2~ (acidic)

HPO3VPO4^3" (basic)

H₂P0²7P0₄ ^3~ (basic)

BH₄7B0₂ ^~ (basic)

S₂0₃ ²7S0₄ ^2~ (basic)

S0₃ ²7S0₄ ^2" (basic)

S₂0₃ ²7S0₃ ^2" (basic)

HC0₂ /CO3^2" (basic)

HS0₄-/S₄0₆ ^2" (acidic)

S₄0₆ ²7HS₂0₃ ^~ (acidic)

S0₄ ²7S₂0₆ ^2" (acidic)

HS0₄7S₂0₆ ^2~ (acidic)

H₂S0₄/HS0₄ ^" (acidic)

S0₄ ²7H₂S0₃ (acidic)

H₂S03/HS₂0₄ ^" (acidic)

S0₃ ²7S₂0₃ ^2" (basic)

S₂0₃ ²7HS^" (basic)

S0₄ ²7HS^" (basic)

S0₃ ²7S₄0₆ ^2" (basic)

S0₄ ²7S₄0₆2-(basic)

S0₄ ²7S0₃ ^2" (basic)

S0₃ ²7S₂0₄ ^2" (basic) Specific Two-Phase Systems

[00198] Any of the acidic discharge catholyte systems described above can be coupled with any of the acidic discharge anolyte systems described above, and any of the basic discharge catholyte systems described above can be coupled with any of the basic discharge anolyte systems described above.

Example of System with Relatively Constant pH

[00199] Often, reactions that include bromates may be run under acidic conditions to minimize solubility problems. The following system may maintain the same acidic pH at the start and finish of the discharge process or at the start and finish of the charge process. This would be advantageous because the energy density may be determined more by the solubility of one of the active species than by the chemical stability of a cation exchange membrane while in contact with a high proton concentration, and less by the minimum pH value. Another advantage is the ability to minimize exposure of a cation exchange membrane to extremely low pH values, which should improve the membrane lifetime.

[00200] Two possible half cell reactions are as follows:

Br0₃ ^~ + 6H⁺ + 6e^~— > Br^"+ 3H₂0 1.44 volts

H₃P0₃ + H₂0— > H₃P0₄ + 2H⁺ + 2e^~ 0.28 volts

The overall cell reaction is:

Br0₃ ^" + 3H₃P0₃— > Br^" + 3H₃P0₄ 1.72 volts

[00201] The starting composition can be made by mixing phosphoric acid with sodium, potassium and/or ammonium bromide salts. The ammonium salt exhibits the best bromate solubility, but adding other salts may aid in the solubility of the various phosphorous containing salts. Because phosphoric and phosphorous acid are both weak acids, the proton concentration will only be a small fraction of the acid concentration.

H₂P0₄ ^" and H₂P0₃ ^" will be present at the same concentration as the protons. HP0₄ ^2", HP0₃ ^2", P0₄ ^3" and P0₃ ^3" will only be present in very small quantities. This is beneficial since the solubilities of the phosphate and phosphite salts are not high and therefore will not hinder the design of a high energy density system. Obviously, the system will have significant complexity as the protons are consumed and generated and interact with the products of the two acid dissociations. In one embodiment, protons are present from the acid dissociations without addition of anions from another acid to the system. In another embodiment, one or more additional acids are added to raise the proton concentration.

Hydrobromic acid may be a beneficial acid to add. This acid would not introduce any additional electrochemically active species since bromide ions are already present. In some embodiments, a complexing agent is added as well. This material dissolves in the aqueous phase and reacts with bromine to form a dense, oil phase. A large number of bromine atoms are complexed by a single complexing agent molecule, for example MEM or MEP. In some embodiments, MEM and MEP are included in equal or non-equal molar amounts. Since there is no bromine present in this single, starting solution, no oil will form and the mixed liquid can be divided between the discharge catholyte tank and the discharge anolyte tank. Alternatively, non-equal amounts of the complexing agents may be used.

[00202] When charging the cell, the anolyte and catholyte may be pumped to their respective compartments. In the charge anolyte, bromide will be oxidized to bromine, HOBr and/or Br(V. If bromine is formed, it will chemically react with the complexing agent(s) and form an oil. When the solution is returned to the discharge catholyte tank, the oil phase will sink to the bottom due to its high density compared to the aqueous phase. If HOBr is formed, it will either be further oxidized to bromate or it will disproportionate into bromine and bromate. Bromide and bromate will react if both are present and chemically form bromine. Just the aqueous phase or an emulsified phase can be pumped to the anode for the first part of the charge reaction. If just the aqueous phase is pumped, all of the bromide will eventually be consumed and some fraction of bromine will be complexed in the oil phase. To complete the reaction, the oil phase must be placed in contact with the cathode along with the aqueous solution. Emulsifying the oil and aqueous phase and pumping that to the anode must be done to complete the conversion of the remaining bromine to HOBr and Br0₃ ^~. IF HOBr forms, it will be further electrochemically oxidized to bromate or disproportionate into bromine and bromate. Eventually, all the bromine will be present as bromate. As the reaction proceeds, the pH will tend to rise as bromide is converted to complexed bromine. The worst case scenario is for one proton to be consumed for every bromide anion oxidized. As the proton concentration decreases, the two acids will generate more protons to maintain their equilibrium due to their weak acid properties. Once the maximum complexed bromine concentration is attained, the pH will start to drop as bromine is oxidized to HOBr and Br03^" and more protons migrate back into the discharge catholyte than electrons traveling in the external circuit. The weak acids may decrease the proton concentration. At the end of the charge reaction, the pH should be very close to the value at the start of the charge.

[00203] In the charge catholyte, phosphoric acid will be reduced to phosphorous acid.

Since the number of electrons equals the number of protons in this half cell oxidation reaction, the pH will remain relatively constant. Two protons will be consumed for each H₃PO₄ molecule that is reduced, but two protons will migrate into the discharge anolyte. Other cations may also migrate. Typically, cation exchange membranes migrate protons at a significantly higher rate than other cations.

[00204] The cell may then be discharged. The charge anolyte and catholyte can be pumped to their respective compartments. In the discharge catholyte, bromate will be reduced to HOBr, bromine and/or bromide. If bromine is formed, it will chemically react with the complexing agents and form an oil. When the solution is returned to the discharge catholyte tank, the oil phase will sink to the bottom due to its high density compared to the aqueous phase. If HOBr is formed, it will either be electrochemically reduced to bromine or bromide or it will disproportionate to bromine and bromate. If bromide is formed, it will react with any bromate present to form bromine. Just the aqueous phase or an emulsified phase can be pumped to the cathode for the first part of the discharge reaction. If just the aqueous phase is pumped, all of the bromate will eventually be consumed and the bromine will be complexed in the oil phase or be present as bromide. To complete the reaction, the oil phase must be placed in contact with the cathode along with the aqueous solution.

Emulsifying the oil and aqueous phase and pumping that to the cathode must be done to complete the conversion of the remaining bromine to bromide. As the reaction proceeds, the pH will tend to rise as bromate is converted to complexed bromine in any number of reaction paths. As the proton concentration decreases, the acids will generate more protons to maintain their equilibrium due to their weak acid properties. Once the maximum complexed bromine concentration is attained, the pH will start to drop as bromine is reduced to bromide and protons migrate back into the discharge catholyte. The weak acids may decrease the proton concentration. At the end of the discharge reaction, the pH should be very close to the value at the start of the discharge.

[00205] In the discharge anolyte, phosphorous acid will be oxidized to phosphoric acid. Since the number of electrons equals the number of protons in this half cell oxidation reaction, the pH will remain relatively constant. Two protons will be generated for each H₃PO₃ molecule that is oxidized, but two protons will migrate out of the discharge anolyte. Other cations may also migrate. Typically, cation exchange membranes migrate protons at a significantly higher rate than other cations.

[00206] At the end of the discharge process, the anolyte and catholyte are very similar. The pH is substantially the same as it was at the beginning of the discharge process. There is virtually no oil in either the anolyte or catholyte since the bromine is

predominantly present as bromide. The phosphorous species is predominantly phosphoric acid in both solutions. The phosphoric acid will associate or disassociate to maintain its equilibrium with protons, H₂P0₄ ^~, HP0₄ ^2~ and P0₄ ^3~ ions. The anolyte and catholyte can now be mixed to produce a substantially uniform composition. Any oil phase present in the discharge anolyte tank can be returned to the discharge catholyte tank using methods discussed above. By doing this, the full capacity of the system is restored and it will be the same for every subsequent cycle following a mixing procedure.

[00207] The energy density will be determined by the concentration of all of the required species in the anolyte and the catholyte. The use of a complexing agent which forms a phase with a relatively high bromine concentration improves the energy density compared to a typical aqueous-only system. Although the pH remains substantially the same at the start and end of discharge or charge, it does exhibit minimum values in the middle of the discharge cycle and the middle of the charge cycle. Generally, the energy density may be limited by the lowest pH value that is compatible with the cation exchange membrane to achieve the desired membrane lifetime. It is less likely to be limited by the solubility of one of the potential salts in this particular system due to the relatively large solubility of most of the salts, taking into consideration the properties of the weak acid properties of the phosphorous-containing acids. It is advantageous to have two half cell reactions that exhibit the same pH at the start and end of the discharge and charge reactions because the time at minimum pH will be minimized. The system can also be engineered to manipulate the reaction path so that the minimum pH is maximized and the time spent at the minimum pH value is minimized. Example of System with Varying pH

[00208] An example of a basic system where the pH cycles between discharge and charge is provided. The two half cell reactions are:

Br0₃ ^" + 3H₂0 + 6e^"— > Br^" +60H^" 0.61 volts\

HC0₂ ^" + 30H^"— > C0₃ ^2" +2H₂0 + 2e^" 0.93 volts

The overall cell reaction is:

BrCV + 3HC0₂ ^" + 30H^"— > Br^" +3C0₃ ^2" + 3H₂0 1.54 volts

[00209] The starting composition can be made by mixing potassium bromide, potassium hydroxide, and potassium carbonate. All of the potential salts in this system are highly soluble with potassium cations. MEM and MEP may be included in equal or non- equal molar amounts. It is possible that there could be no significant quantity of bromine formed in this system due to the tendency of bromine to disproportionate at high pH values. Alternatively, some bromine may form but may not disproportionate rapidly, so it would be desirable to complex the bromine to reduce the bromine vapor pressure from the aqueous phase. The mixed liquid can be divided between the discharge catholyte tank and the discharge anolyte tank. Typically, equal quantities are used, but it is not a requirement to do so.

[00210] For charging, the anolyte and the catholyte can be pumped to their respective compartments. In the charge anolyte, bromide will be oxidized to bromine, BrO^" and/or Br0₃. If bromine is formed, it will chemically react with the complexing agents and form an oil or it will disproportionate to bromide and BrO^". When the solution is returned to the discharge catholyte tank, any oil phase will sink to the bottom due to its high density compared to the aqueous phase. If BrO^" is formed, it will either be further oxidized to bromate or it will disproportionate into bromide and bromate. Unlike an acidic system, bromide and bromate will not react if both are present in a basic system. Just the aqueous phase or an emulsified phase can be pumped to the anode for the first part of the charge reaction. If just the aqueous phase is pumped, all of the bromide will eventually be consumed and some fraction of bromine will be complexed in the oil phase. To complete the reaction, the oil phase must be placed in contact with the cathode along with the aqueous solution. Emulsifying the oil and aqueous phase and pumping that to the anode must be done to complete the conversion of the remaining bromine to BrO^" and Br0₃ ^". If BrO^" forms, it will be further electrochemically oxidized to bromate or disproportionate into bromide and bromate. Eventually, all the bromine will be present as bromate. As the reaction proceeds, the pH will tend to rise as bromide is oxidized, if any species other than bromate is formed. Once the maximum complexed bromine plus BrO^" concentration is attained, the pH will start to drop as bromine and BrO^" are oxidized to BrO^" and Br0₃ ^" and more hydroxyl ions are consumed than migrate back into the discharge catholyte. The pH will be approximately the same at the start and finish of the charging process in the charge anolyte.

[00211] In the charge catholyte, carbonate will be reduced to formate. Since the number of hydroxyl ions exceeds the number of electrons, the pH will rise during the charge process. Other anions may migrate in addition to hydroxyl ions. Typically, anion exchange membranes migrate hydroxyl ions at a higher rate than other anions. The maximum pH value will be attained at the end of the charge process. The maximum pH value that is compatible with the anion exchange membrane will be determined by the pH value in the charge catholyte at the end of the charge cycle and determine the maximum energy density of the system.

[00212] When discharging the system, the anolyte and the catholyte can be pumped to their respective compartments. In the discharge catholyte, bromate will be reduced to BrO^", bromine and/or bromide. If bromine is formed, it may chemically react with the complexing agent(s) and form an oil or disproportionate to Br^" and BrO^". When the solution is returned to the discharge catholyte tank, the oil phase will sink to the bottom due to its high density compared to the aqueous phase. If BrO^" is formed, it will either be

electrochemically reduced to bromine and/or bromide or it will disproportionate to bromide and bromate. Eventually, all the bromine will be present as aqueous bromide. Just the aqueous phase or an emulsified phase can be pumped to the cathode compartment for the first part of the discharge reaction. If just the aqueous phase is pumped, all of the bromate will eventually be consumed and the bromine will be complexed in the oil phase or be present as bromide. To complete the reaction, any oil phase must be placed in contact with the cathode along with the aqueous solution. Emulsifying the oil and aqueous phase and pumping that to the cathode must be done to complete the conversion of the remaining bromine to bromide. As the reaction proceeds, the pH will tend to rise as bromate is reduced to any species other than bromide. Once the maximum complexed bromine plus BrO^" concentration is attained, the pH will start to drop as bromine and BrO^" are reduced to BrO^" and Br^" and more hydroxyl ions are consumed than migrate back into the discharge catholyte. The pH will be approximately the same at the start and finish of the discharge process in the discharge catholyte.

[00213] In the discharge anolyte, formate will be oxidized to carbonate. Since the number of hydroxyl ions exceeds the number of electrons, the pH will drop during the discharge process. Other anions may also migrate in addition to hydroxyl ions. Typically, anion exchange membranes migrate hydroxyl ions at a higher rate than other anions. The pH value at the end of the discharge cycle in the discharge anolyte will be very close to that of the discharge catholyte.

[00214] At the end of the discharge process, both electrolytes are very similar in concentrations and pH. There will be virtually no oil in either one since the bromine is predominantly present as bromide. The carbon species is predominantly carbonate. The anolyte and catholyte may be mixed to produce a substantially uniform composition. Any oil phase present in the discharge anolyte tank can be returned to the discharge catholyte tank using methods discussed above. By doing this, the full capacity of the system is restored and it will be the same for every subsequent cycle following a mixing procedure.

[00215] The energy density will be determined by the maximum pH in the charge catholyte at the end of the charge cycle. A maximum of one hydroxyl ion will migrate into the charge anolyte for each bromide ion that is oxidized to complexed bromine. After most of the bromide is converted, the pH will generally go down. Two hydroxyl ions will migrate out of the charge catholyte while three hydroxyl ions will be created for each reduced carbonate ion that forms a formate ion. Since there will be three times more carbonate species than bromide species taking place in the charge reactions, it is the carbonate reduction reaction that will determine the maximum pH. The initial, substantially uniform solution can start with a relatively small amount of hydroxyl ions, just enough to provide adequate conductivity and prevent any significant concentration overpotentials. Unfortunately, anion exchange membranes are typically less stable at a high hydroxyl ion concentration compared to cation exchange membranes at the same proton concentration, so this will be a factor in determining the maximum, practical energy density.

[00216] Although the foregoing invention has been described in some detail by way of illustration and examples for purposes of clarity of understanding, it will be apparent to those skilled in the art that certain changes and modifications may be practiced without departing from the spirit and scope of the invention. Therefore, the description should not be construed as limiting the scope of the invention.

[00217] All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entireties for all purposes and to the same extent as if each individual publication, patent, or patent application were specifically and individually indicated to be so incorporated by reference.

Claims

WHAT IS CLAIMED IS:

1. A secondary battery, comprising two compartments, wherein a first compartment comprises an anolyte plus anode and a second compartment comprises a catholyte plus cathode, wherein the first and second compartments are divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte and permits an ionically conducting path between the anode and cathode;

wherein the battery comprises an overall cell reaction that comprises an anode half cell reaction and a cathode half cell reaction;

wherein prior to charging, the anolyte and the catholyte comprise substantially the same liquid composition that predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction, wherein the liquid composition comprises a basic pH;

wherein upon charging the secondary battery:

in the charge anolyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and

in the charge catholyte, the product of the discharge anode reaction is reduced to a lower oxidation state, and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged;

wherein upon discharging the secondary battery:

in the discharge catholyte, the product of the discharge cathode reaction is in its lowest oxidation state or the lowest oxidation state achieved during the charging and discharging of the battery, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and

in the discharge anolyte, the product of the discharge anode reaction is in its highest oxidation state or the highest oxidation state achieved during the charging and discharging of the battery and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged;.

wherein optionally after predominantly discharging the secondary battery:

the anolyte and catholyte are mixed to restore the initial capacity of the system and the mixed composition predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction.

2. A secondary battery according to claim 1, wherein the liquid composition is aqueous.

3. A secondary battery according to claim 1, wherein the first compartment and the second compartment are separated by an ion exchange membrane.

4. A secondary battery according to claim 1, wherein the battery is a flow battery.

5. A flow battery according to claim 4, wherein the catholyte and anolyte are mixed periodically prior to charging the battery.

6. A secondary battery according to claim 1, wherein the discharge cathode half cell reaction comprises reduction of a halogen oxyanion.

7. A secondary battery according to claim 6, wherein the halogen in the halogen oxyanion is bromine, chlorine, iodine, or a mixture thereof.

8. A secondary battery according to claim 6, wherein the halogen oxyanion is bromate, perbromate, hypobromite, or BrO^".

9. A secondary battery according to claim 6, wherein the liquid composition comprises a complexing agent.

10. A secondary battery according to claim 1 , wherein the discharge cathode half cell reaction comprises reduction of a manganese or chromium oxyanion.

11. A secondary battery according to claim 1 , wherein the anode half cell reaction comprises oxidation of a tin oxyanion.

12. A secondary battery according to claim 1 , wherein the discharge anode half cell reaction comprises oxidation of phosphite, hypophosphite, phosphorous acid,

hypophosphorous acid, or a sulfur containing oxyanion.

13. A secondary battery according to claim 1 , wherein the product of the discharge cathode half cell reaction in the catholyte comprises more than one catholyte active species that is oxidized to a higher oxidation state during charging of the battery, and wherein none of the catholyte active species reacts chemically with any other component of the catholyte when the battery is charged or discharged.

14. A secondary battery according to claim 1 , wherein the product of the discharge anode half cell reaction in the anolyte comprises more than one anolyte active species that is reduced to a lower oxidation state during charging of the battery, and wherein none of the anolyte active species reacts chemically with any other component of the anolyte when the battery is charged or discharged.

15. A secondary battery according to claim 14, wherein the product of the discharge cathode half cell reaction in the catholyte comprises more than one catholyte active species that is oxidized to a higher oxidation state during charging of the battery, and wherein none of the catholyte active species reacts chemically with any other component of the catholyte when the battery is charged or discharged.

16. A secondary battery, comprising two compartments, wherein a first compartment comprises an anolyte plus anode and a second compartment comprises a catholyte plus cathode, wherein the first and second compartments are divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte and permits an ionically conducting path between the anode and cathode;

wherein the battery comprises an overall cell reaction that comprises an anode half cell reaction and a cathode half cell reaction;

and wherein the discharge anode half cell reaction comprises phosphate/phosphite, phosphate/hypophosphite, phosphoric acid/phosphorous acid, phosphoric

acid/hypophosphorous acid, formate/carbonate, or sulfur oxyanion/sulfur oxyanion.

17. A secondary battery according to claim 15, wherein prior to charging, the anolyte and the catholyte comprise substantially the same liquid composition that predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction;

wherein upon charging the secondary battery:

in the charge anolyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and

in the charge catholyte, the product of the discharge anode reaction is reduced to a lower oxidation state, and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged;

wherein upon discharging the secondary battery:

in the discharge catholyte, the product of the discharge cathode reaction is in its lowest oxidation state or the lowest oxidation state achieved during the charging and discharging of the battery, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and

in the discharge anolyte, the product of the discharge anode reaction is in its highest oxidation state or the highest oxidation state achieved during the charging and discharging of the battery and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged;.

wherein optionally after predominantly discharging the secondary battery:

the anolyte and catholyte are mixed to restore the initial capacity of the system and the mixed composition predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction.

18. A secondary battery according to claim 16, wherein prior to charging, the anolyte and the catholyte comprise substantially the same liquid composition.

19. A secondary battery according to claim 16, wherein the battery is a flow battery.

20. A flow battery according to claim 19, wherein the discharge catholyte and anolyte are mixed prior to charging the battery such that the anolyte and catholyte comprise substantially the same liquid composition prior to charging.

21. A secondary battery according to claim 16, wherein the product of the discharge cathode half cell reaction in the catholyte comprises more than one catholyte active species that is oxidized to a higher oxidation state during charging of the battery, and wherein none of the catholyte active species reacts chemically with any other component of the catholyte when the battery is charged or discharged.

22. A secondary battery according to claim 16, wherein the product of the discharge anode half cell reaction in the anolyte comprises more than one anolyte active species that is reduced to a lower oxidation state during charging of the battery, and wherein none of the anolyte active species reacts chemically with any other component of the anolyte when the battery is charged or discharged.

23. A secondary battery according to claim 22, wherein the product of the discharge cathode half cell reaction in the catholyte comprises more than one catholyte active species that is oxidized to a higher oxidation state during charging of the battery, and wherein none of the catholyte active species reacts chemically with any other component of the catholyte when the battery is charged or discharged.

24. A secondary battery, comprising two compartments, wherein a first compartment comprises an anolyte plus anode and a second compartment comprises a catholyte plus cathode, wherein the first and second compartments are divided by a membrane or separator that substantially prevents mixing of the anolyte and catholyte and permits an ionically conducting path between the anode and cathode;

wherein the battery comprises an overall cell reaction that comprises an anode half cell reaction and a cathode half cell reaction; and

wherein the discharge anode half cell reaction is selected from phosphate/phosphite, phosphate/hypophosphite, phosphoric acid/phosphorous acid, phosphoric

acid/hypophosphorous acid, sulfur oxyanion/sulfur oxyanion, formate/carbonate, sulfur oxyanion/sulfur oxyanion, or a metal ion, and wherein the discharge cathode half cell reaction comprises halogen oxyanion/halide, Mn0₄7Mn²⁺, or

HCr0₄7Cr²⁺.

25. A secondary battery according to claim 24, wherein prior to charging, the anolyte and the catholyte comprise substantially the same liquid composition that predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction;

wherein upon charging the secondary battery:

in the charge anolyte, the product of the discharge cathode reaction is oxidized to a higher oxidation state, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and

in the charge catholyte, the product of the discharge anode reaction is reduced to a lower oxidation state, and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged;

wherein upon discharging the secondary battery:

in the discharge catholyte, the product of the discharge cathode reaction is in its lowest oxidation state or the lowest oxidation state achieved during the charging and discharging of the battery, and the product of the discharge anode reaction is electrochemically inactive and remains unchanged; and

in the discharge anolyte, the product of the discharge anode reaction is in its highest oxidation state or the highest oxidation state achieved during the charging and discharging of the battery and the product of the discharge cathode reaction is electrochemically inactive and remains unchanged;

wherein optionally after predominantly discharging the secondary battery:

the anolyte and catholyte are mixed to restore the initial capacity of the system and the mixed composition predominantly contains the products of the discharge cathode reaction and the products of the discharge anode reaction.

26. A secondary battery according to claim 24, wherein the discharge cathode half cell reaction comprises a halogen oxyanion and the halogen in the halogen oxyanion is bromine, chlorine, iodine, or a mixture thereof.

27. A secondary battery according to claim 26, wherein the halogen oxyanion is bromate, perbromate, hypobromite, or BrO^", and the halide is bromide.

28. A secondary battery according to claim 24, wherein prior to charging, the anolyte and the catholyte comprise substantially the same liquid composition.

29. A secondary battery according to claim 26, wherein the anolyte and catholyte further comprise a complexing agent.

30. A secondary battery according to claim 24, wherein the product of the discharge cathode half cell reaction in the catholyte comprises more than one catholyte active species that is oxidized to a higher oxidation state during charging of the battery, and wherein none of the catholyte active species reacts chemically with any other component of the catholyte when the battery is charged or discharged.

31. A secondary battery according to claim 24, wherein the product of the discharge anode half cell reaction in the anolyte comprises more than one anolyte active species that is reduced to a lower oxidation state during charging of the battery, and wherein none of the anolyte active species reacts chemically with any other component of the anolyte when the battery is charged or discharged.

32. A secondary battery according to claim 31 , wherein the product of the discharge cathode half cell reaction in the catholyte comprises more than one catholyte active species that is oxidized to a higher oxidation state during charging of the battery, and wherein none of the catholyte active species reacts chemically with any other component of the catholyte when the battery is charged or discharged.

33. A liquid composition for a secondary battery, comprising a discharge catholyte active species comprising a halogen oxyanion, a manganese oxyanion, or a chromium oxyanion, and an anolyte active species comprising a phosphite, hypophosphite, phosphorous acid, hypophosphorous acid, sulfur oxyanion, formate, or metal ion.

34. A composition according to claim 33, wherein the discharge catholyte active species comprises a halogen oxyanion and the halogen in the halogen oxyanion is bromine, chlorine, iodine, or a mixture thereof.

35. A composition according to claim 34, wherein the halogen oxyanion is bromate, perbromate, hypobromite, or BrO^".

36. A composition according to claim 34, further comprising a complexing agent.

37. A composition according to claim 33, wherein the composition is aqueous.